I would do a splice in the middle, then heat treat any reflex/deflex you may want into the Osage first, then glue the bamboo backing on.
Unless your just doing a mild reflex, then it wouldn't be necessary to preshape the osage.
It's cheap insurance so it doesn't bend in the handle on ya. I don't think the bamboo would have enough backbone to support the method you mentioned anyway?
There are some good splice alongs here, I just did a BBO myself with two billets spliced in the handle and it worked out great.
Do your best to get the boo backing as flat as possible and try to keep it even thickness from end to end because it will make tillering it much easier on down the road.
Eric Krewson did a BBO repair along where he sanded off the damaged boo backing and replaced it with a new one. You can use that for gluing the boo onto the osage lam when you get to that point, it's the one I used and it worked out perfectly.
http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=005458;p=1