I say go for it if you're worried about sapwood integrity! It's clearly a material the bowyers would have had access to in those times. You usually don't actually need to back yew sapwood even with violated rings - I've got an old yew warbow/longbow hybrid with horrible ring violations and knots on the edge and black streaks of rot running through and it still shoots at 32" with no problem.
There is no evidence either documented or on any of the finds of any yew warbows of backing, using any material, but that's not to say it wasn't used I guess?
Personally I don't like it, and it wouldn't pass the specs if you wanted to shoot one at an official warbow shoot, but if it's just an experiment or you prefer them, why not?!