Well...........actually I have a new discovery I've used just once so far and it's good.
"Bear Stuff" dry wash from Buildabear comes in a pump spray for $8 and is wet for a short while then you scrub it and it becomes foam and you wipe it off...on the plushes for surface cleaning. It was suggested to use on their hair and I tried it out two days ago on a really tangled Bab Celestia from the thrift store and it worked very well. Just use enough to moisten the area you're working with and reapply as it dries.
I had a much worse condition Bab Fluttershy a while ago that I tried diluted fabric softener on, and that worked well but I had to tie the thing hanging upside down to keep it from running onto the plush and it was tough on the hands after a while.
Really any conditioner etc will work or even just hot water if you are OK with immersing it, but if you want to keep the plush dry I think the best stuff is the Bear Stuff.
I'm working on a G3 Styling Cheerilee I just got today at thrift, and while I started with the Bear Stuff, it was using it up too fast so and isn't necessary on a non-plush so I'm using conditioner and her hair is much finer and tougher to work than the Bab plush hair. I think most plush hair is quite a bit heavier than the plastic ponies.
For hot water, just fill up a big bowl and hang the plush over it and keep dunking the hair. Use a rubber band or something to put a loop on the feet and hang from something above a drip-collector of some kind so you can use both hands on the hair.