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.
Kokyu and ki would come up all the time. I used to train in the Aikikai and at an unaffiliated dojo, and these were the core, fundamental ideas. I also recently visited a Roppokai (Daito Ryu) dojo, and we did some agete, and did some 'kokyu' work that would help 'tilt' the pelvis, stretch the back suit, suck the guy in and take his balance--there was talk about 'kokyu' power again but not about the pelvis, although it was being worked directly.
Maybe these fellas are just keeping the secrets from me?