I've had a mind to do something like this for a while but haven't pulled the trigger. I think the real success, and this is just hypothesis, comes from thinking of "finishing" rather than "aging". This probably works better if the barrel has been used once or twice as well, so the intensity of the new wood and small surface area are mitigated. I would probably age something I wasn't going to sip straight to begin with, like white rum. If I ruin 3L of Bacardi, it's just going to get mixed in with coke, no big deal. Then 2nd fill, maybe some local moonshine - give it a little color, and maybe get some kind of bizarre AE Rye thing going on from the rum (but probably not). Then I would think the barrel would have mellowed enough to try some real "finishing" of straight whiskies. Maybe barrel aging the SB blend (which has been mentioned here on the board before). Eventually the barrel would become mostly dormant and would probably be a good vessel for doing cocktail mingling (which has also been mentioned here many times).