Everyone knows you can't make a bow out of osage, just can't be done! Especially an ELB! Nope. Never.
But since you are going to do it anyway, backing the board with something like 'boo or hickory would be the ticket. It's going to go a long way to fix the issue with grain violations. The 'boo osage appeals most to me because of the contrast in coloration, referencing the sapwood/heartwood combination of classic yew longbows.
'Sage has plenty of compression strength, so no worries about the backing overpowering the belly, either. But being dense wood, you may want to make sure that your limb tips are plenty thin to counter the possibilities of hand shock.