I guess a truely primitive bow would have rawhide or sinew backing. For your first bow you ought to concentrate on proper tiller and the proper use of the tools to get you there. I'd bet this won't be the only bow you will build. You can always learn to be more primitive later.
Make a good shooting bow and be proud of it, then go start your next. If your first bow fails, and most do
, learn from your mistakes and start another.
With osage, if the rings are decent early/late ratio and you chase a back ring with no violations in it you should not need a backing. Also, if you screw up that nice snaky osage stave you will hate yourself in the morning.
There will come a time when you wished you had it back again!
I know this for a fact!!!
Pat