Are we talking a homebuilt or a certified airplane?
On my Fly Baby, there's a socket in the belly that I can plug a battery charger into. It's the round thing on the top left of this picture:
This is actually a standard 240V socket, mounted kinda flush with the bottom of the airplane. There's a 240V plug in it in the picture...not hooked up to anything, just occupying the holes to keep them clean. I've got a matching plug on the output of my battery charger, and can just pop off the blank plug and plug in the battery charger.
If you've got a certified airplane, you could look into some of the STCs for external power jacks. You might even be able to have your IA install one on a Form 337.
Ron Wanttaja