With such a large volume, I would recomend a Vanilla Mead.
Mmm, with Buckwheat honey.
The whisky flavoring and a bit of the oak will bleed through to the flavor. Whisky is usually out of a medium or heavy toast oak. So you will get some carmelly and possibly smoky notes from this. So think along those lines. Hmm, actually with peach season here a Whisky peach sounds great. Just a bit of vanilla, whisky and in the secondary put in some of the maple syrup. Also some nutmet or cinnamon to spice it up a bit.
That sounds good. But seriously, any mead would be good out of it. Just expect the barrel to give a little of the whisky flavor and a touch of the carmelly smokyness from the oak. Should be good.