I haven't, but boy you could could get something interesting going with a stout. Especially if you aged it on heavy toasted oak soaked in red wine/port/sweet vermouth. Could through a pound of cherries in there too...
I've never added into the bulk beer, but I've used cherry bitters to spike a batch of cherry Berliner as served in the glass which I felt was lacking punch.
Go hard man! Experiment! You might piss of some style elitists in the process but who cares about them? Have fun and enjoy it! Try new things and don't be afraid!