In answer to whether or not ikkyo would work in MMA yes
I know this as i used it the other day myself. I did not use ikkyo to completely pin and control my partner if that what is meant by the original thread. However I was facing a guy who was around 5 stone heavier than me and be was burning himself out trying to apply a headlock & choke which he didnt have.
So i just sat there and practiced some relaxed breathing kept my chin tight to my chest, and when i felt an opening caught the wrist he was applying the choke with and reached up behind my head to push his elbow forward. I only meant it as a means of escape from the choke however he because he wasn't used to an ikkyo he didnt even put his free hand out to stop himself and went head first into the floor.
I have also found it a useful escape when someone pounces trying to take your back, but you have to be fast before they get their hooks in.
So as previously stated by others ikkyo does work it just may look a little bit different than it does in the dojo with a flowing partner.
Also people mentioning sankyo and nikkyo in MMA, i'm fair sure that you don't see them used ever because its against the rules of most UFC style fights to use wrist (and finger and toe holds for that matter.)