Can one make photoshop plugins with bmax?

Yep... PS plugins are DLLs renamed to 8bf, you can request a free SDK or try some commercial generator via the list.

http://www.thepluginsite.com/knowhow/plugincreation.htm

I remember it's possible to do this with MinGW at least in PhotoShop CS, but is troublesome and I don't have any intention to try again.