I don't see why black locust would make a 100# English longbow, nut not a 100# flatbow. If you're getting more set with a flatbow in that weight range than with a longbow, there must be something wrong in your design. The tiller could be off, for instance. Or, what I think is more likely, the flatbow is not long enough. It's pretty futile to try a 120#@28" flatbow out of a 64" stiff handles bow. The bow must be longer to accommodate for the higher drawweight, resulting in equal strain. 74" would not be excessive, in my humble opinion. What would be wrong with that extra length?
Black locust isn't exactly a cousin of osage. While osage is in the mulberry-family, botanically speaking, the black locust is in the pea-family (along with true rosewoods and jatoba, to name a few). Black locust just 'happens' to looks a lot like osage, and also many mechanical properties a pretty much alike, albeit a bit lower in value.