Thank you very much for the series, and this article in particular, which I'm finding helpful in both form and substance, given what I'm supposed to be working on right now. Even so, this is arguably a diversion, so I will be brief.
1) The translation of "inkyo" as "enlightenment" is more than a bit suspect. Although "inka" and "kensho" are widely used as synonyms for "enlightenment" in popular literature on Rinzai Zen, "inka" actually certifies "completion of a program of (usually koan) study." What is much more likely is that "inkyo" is a formal license or "permission," in this case, specific permission to practice the rite of Daigensui Myoo. A second possibility (these two are not mutually exclusive) is that "inkyo" is a document containing specific instructions on the various elements of the practice of Daigensui Myoo.
2) Daigensui Myoo
has a very interesting and comparatively well-documented history. Most importantly, he is a comparatively low-order deity, classed as a "yaksha" or "dharmapala," i.e. "dharma protector." Given the context of that initiation -- the Russo-Japanese War -- and the historic association of Daigensui Myoo with the suppression of threats to the Imperial Household, it seems likely that this initiation was widely practiced at that particular moment, as it had been in previous periods of war and internal insurrection.
In the course of working on my dissertation, I've come into contact with the correspondence between Minakata Kumagusu and Toki Horyu, then Dai-Ajari at Koyasan. As Gerald Figal writes in Civilization and Monsters:Spirits of Modernity in Meiji Japan
, Duke University Press, 1999:
Part of Minakata's project in these letters written from London was to convince Toki of the compatibility of Mahayana Buddhism with modern science and to dissuade him from relying on the mystic and superstitious side of Buddhism to fuel its revival in Meiji Japan.
But this isn't as simple as it might seem at first glance. Parts of Minakata's correspondence with Toki and his other writings treat some matters we might be inclined to see as the "superstitious side of Buddhism" as "embodied knowledge" expressed in a different language than that of western empirical science, and thus, in need of translation rather than ripe to be jettisoned. Wherever one comes down on the question of where such practices as that of Daigensui Myoo fall on that spectrum, what we can be reasonably sure of is that the initiation into practice of Daigensui Myoo has a long cultural provenance in Japan and a very specific association with Shingon's official imperial remit: "chingo kokka -- ‘pacifying and defending the nation'." Taking all of these elements into account, the likelihood that the "inkyo" in question is much more than documentation of an almost pro forma initiation intended to give comfort to a lay Shingon practitioner who is going off to serve the Emperor in war becomes almost nil. That doesn't, however, address how this particular practitioner experienced the initiation, particularly in light of Ueshiba's self-reported robust mimicry of priestly Shingon ritual practice, which brings us to our next item.
3) It should also be noted that Shingon holds many of its principal rites very closely, and permission to perform these rites is tightly restricted to ordinants. It would be, in a word, astonishing
for a lay Buddhist to be granted a certificate of "enlightenment" in Shingon, and it would be even more astonishing for such a certificate to be issued on the occasion of initiation into the practice of a lower-order demi-god such as Daigensui Myoo.
4) This creates an interesting parallel between some of the conditions that obtained during the rise of Tantric Buddhism in India and those in which Ueshiba found himself. In a nutshell, many basic tantric rites were originally restricted to rulers of appropriate family background. This created a problem in (for example) the Gupta Dynasty, because the King had the obligation to perform certain rites (or at least, his subjects had a cultural expectation that he would perform them), but lacked the appropriate Brahmanic family background to gain access to the teaching in a historically approved fashion. The solution was to transfigure the rites from their Vedic context to a Buddhist context which had dropped the varna restrictions.
Short of becoming a priest, Ueshiba's access to higher teachings in Shingon was blocked. With regard to what we now term "Shinto" practice, Ueshiba's access to a number of rites was similarly blocked because of his inappropriate family background. Deguchi's Oomoto offered a solution to that dilemma, precisely by offering devotees a similarly complex and conceptually related (though distinct) body of theory, a principle of "universal priesthood" for Oomoto believers, and a mode of "esoteric" initiation into rites adopted or adapted from Shingon, Tendai, Shinto, and Shugendo lineages which were much more restrictive in regard to permission to not merely observe, but perform, such rites.
From the perspectives of these other traditions, the material in Oomoto is "incomplete" or "lacking appropriate lineage" or simply "not Shingon" or "not Tendai" or what-have-you. These characterizations are typically made in much the same way that members of the Kashima Shinryu have characterized the kenjutsu of Inaba Sensei, with similar, or greater degrees of vehemence.
5. Finally, while it is far from complete, Frederick Victor Dickins' Primitive & Mediaeval Japanese Texts
is now available online through Google Books. The companion volume, with romanization of the kana readings of the included texts is available for purchase elsewhere online for those so inclined. Included in those two works (Item 68, pp. 60-61 in the Kessinger Publishing reprint of the 1906 Clarenden Press companion volume of romanized kana text, pp. 226-228 in the downloadable .pdf at Google Books, pp. 97-99 in the original 1906 Clarenden Press edition.) are the Dickins/Minakata(?) translation of the poem you reference as Book 5, Poem 894.
Along with my earlier thanks, please accept my gratitude for providing an occasion to detail a piece of the work at hand; I feel that I've been both directly on-point and pleasantly diverted this morning, which is a rare combination, though I see that I have utterly failed in my attempt to keep this short.
Nonetheless, I hope this is of some use.