All it would show is that we are different species by the successful interbreeding criteria. No one disagrees that that barrier is there.
The supposed barrier between "kinds" is somehow able to stop a series of speciation events being able to create groups of species that are as different as some are now.
Even if it actually could produce a viable humanzee that was brought to term would it falsify the 'kind' barrier idea? I still don't know what a 'kind' is? It would just mean that we and chimps are the same 'kind' but only by some definitions. So the creationists would redefine kind in some way. For example, they would decide that it only when the interbreeding was natural. And, of course, any interbreeding between a human and a beast is, by any definition they would accept, "unnatural" and an abomination.
So the experiment would end up proving nothing to either side.
Aside from that, I am uneasy enought with the idea that I do not think it should be performed anyway. I have two areas of feeling that lead me to this:
1) This is just an emotional, unsupported distaste for the idea.
2) The other is the possiblity of producing a whole population of humanzees. I don't want to give us another chance to create slaves thank you very much.