Forgive if I'm mistaken, because I don't use iOS 8, but surely there the edits are on JPGs, and the "plug-in" delivers back a new version ? To work in a RAW pipeline, you'd need to provide various registration points for plug-ins, for example MegaDemosaicerPro could register itself as a RAW renderer, MaxVividityPlus as a colour editor and CuttingEdgeX as an output sharpener. And you'd need to assemble your own pipeline for each photo.
Maybe I'm not Thinking Different enough, but I can't see another way to do it. And, according to Derrick Storey, who is on record (well, podcast) as saying he got it from Apple directly, Photos, at least for now will NOT actually implement a extendibility scheme. The marketing intent is to replace iPhoto.
Not that I'm happy about this. I spent an hour or so in Lr5 today, and I don't feel well now :-( Took me 10 mins to find the straighten tool. What a dog's breakfast of a UI (and I'm not totally new to Lr, I used v1 prior to Ap2). I cannot imagine why it is so popular. Fabulous marketing, I guess, helped by Apple arrogance.