Umm, your best answers would probably be from the Only Allman Brothers Thread in the Classic Rock forum, so post there

The only tuning I know Dickey Betts using is standard (doesn't mean that's the only one, though!). And while Duane might have used standard sometimes, Little Martha doesn't use slide but it is in open-E, so he would sometimes use that tuning even when not using the slide.

(Slightly outdated) Electronic and classical compositions by m'self: Check 'em out