I haven't encountered it. Not in any of Ueshiba's doukas, not in Shioda's "Dynamic Aikido", nor did any of my former Aikido teachers say anything important about the pelvis. I didn't mean to suggest that these men 'didn't' talk about it, it's just not prevalent in public literature (excuse my poor choice of words). It's been a while since I read Tohei's books on "Ki" but for the most part he, and Shioda, and Ueshiba talked a lot about 'breath' and 'breath training'. Usng the pelvis is just not a 'fundamental' or 'well known' concept as Buck describes it.
Lately, since the 'internal strength' thing is raging now in the American aiki community, discussions on the pelvis have been popping up, but even then, ideas on how to use it and how it affects the body structure, the fascia, etc. are still, I believe, underdeveloped and unclear, contrary to what Buck ignorantly supposes.
Strange then that my late-aikido teacher here in Japan (Aikikai) talked about "koshi" usage, as did my old teacher back in the US (Yoshinkan), as do just about ever teacher of every other Japanese martial art I have trained in here. I'd go so far as to say that one of the few things you can rely upon Japanese martial artists to stress endlessly, almost regardless of the art, is "koshi" and "kokyu". Whether they actually know what they are talking about is a different issue, but the emphasis is spread across the board pretty evenly. Off hand I would even go as far as to say that having "weak koshi" is one of the most common comments I hear sensei give to their students over here.
As for it not popping up in douka, that isn't really surprising if you have had much exposure to martial poetry in Japan and the sort of topics they tend to cover.