Yeah, great idea. These will work, as long as you can get the appropriate voltage.
Because AA batts are the same voltage as D batts (1.5V for disposables, and 1.2V for rechargeables) this idea will work.
BUT.......
What suffers, is energy storage capacity. It's obvious that you won't be able to run a box (or anything else for that matter......) as long as you would on D batts.
Of course, if storage capacity isn't a problem, or an issue, then yes, this is a good idea.
Also, I would seem to think (depending on your battery re-charger) you would be able to recharge AA batts quicker than D batts.
-BoomBoxDeluxe.