Could you use a pocket diaper insert or extra soakers from a quick dry AIO layed in a close fitting wool cover to make a faux wio? Any do this? Does it work okay or do you need something specifically shaped to fit a particular cover for them to be effective?
Well, WIO really just means a coordinated wool cover w/ a diaper/absorber within it. I think almost all WIOs have covers that can be used as often as a "generic" wool cover with various things within it. I have a BBH and SP that fit this description: both are covers w/ innards that can snap in/out. But when the innards are getting laundered, I still use the cover over fitteds and snappied PFs. As C still has rather random, bf blowouts I can do a plain trifold'd PF in any cover so we always have to "close up" the diaper. I usually just use my BBH as a cover for now for this reason (the "diaper" of the BBH is contoured, but no elastic so it doesn't hold his poos in), whereas the snap-in for the SP is elasticized so holds things better. I'm just trying to avoid poop getting on the covers as much as possible. (did I understand your question wrong?)
I've found joeybunz and the like not to work well for us as lay ins. They're too narrow and scrunch up. If you were going to do it it'd have to be a very snug cover and they dont give a lot of bottom coverage. If you had number 2's happening the cover would certainly get dirty.
That said, I've been using our LCH hemp contours in some wool covers and they're working well. But they're a totally different shape to an aio soaker. And the cover we've been using is actually too small for her to fit over a fitted anymore.