That's a fantastic idea. I like it a lot...
Now someone said about allowing for a diaper butt, well I'm imagining it and a diamond panel in the back should work just fine. I also wanted to add that diamond pannels in the underarms would probably help out in flexability. Seeing it cut out reminded me of many years ago when my grandmother had to fix the T shaped camo shirts a neighbor made as costumes for her kids. She didn't allow near as much room as you did though. I also have had many shalwar kameez, and made some too. those lil diamonds add a lot in the under arm.
This whole post makes me want to run to the thrift store in addition to my planned trip to hancocks today

Wish me luck.
