Thanks for the suggestions. I had considered both Dark and Wood Elves, but neither really have the full mask that Harlequins are supposed to have.
I thought the Tomb King range might have something, but if it does I missed it. The other obvious models with masks are Sanguinary Guard, but they wouldn't really suit.
I'm assuming that the best option will involve what's intended to be a real face as a mask. I did consider a Gnoblar head - might look ok on one, but not sure I'd want them all like that, and then I need to find a range of unique heads...