Oh look, it's the answer I was mistakenly giving on the Melira one!
Putting things onto the battlefield does not count as casting them. So if Ulamog said "When it enters the battlefield" it would work, but since it needs you to cast it, it doesn't work when you Mayael it into play.
awesome thanks, I suspected as much. Definitely good to know before I acquired a $25+ card for a Mayael deck :P
Man, the first EDH deck I ever made was a Mayael deck, and I had the worst luck you have no idea. I think I went something like 12 consecutive fail-to-finds across probably 20 games, most of which I wasn't allowed to untap with her already in play.
It was the moment I decided that if I needed to play a general, and have it still be alive the next turn before I could do anything with it, I'd just not use it.