Peter A Goldsbury
I have asked Doshu and other Hombu teachers whether Morihei Ueshiba did IP training and the answer was yes, but with the rider that he never taught it: he left this type of training to students who perceived it and wanted to do it. The corollary was (is) that this type of training should be a complement to one's 'kihon' training, but not a substitute for it.
I suppose if Morihei Ueshiba did not teach his IP method, then that could account for the eclectic sources for IP training, e.g. yoga, other branches of Daito-ryu, among prominent historical aikidoka -- i.e. it doesn't seem that those prominent folks "who perceived it" simply adopted Morihei Ueshiba's empty-hand and weapon-based aiki-taiso one for one (though why there isn't a single well known case of an IP adept within aikido doing so piques my curiosity, given that Ueshiba openly demonstrated if not expressly taught his method; but perhaps you and others who are well versed in aikido history can address that).
In any case, did Doshu and the other Hombu teachers indicate an acceptance that extant IP training continues via sources outside of the Aikikai (and/or has such a thing as sanctioned IP training expressly designed as a complement to kihon training internal to the Aikikai evolved, even if relatively clandestine and limited to individuals vs. being system wide)?