Guitar only, ... any key besides the flat ones.
I can do it in a pinch, ... but it just ain't natural!
When working with Horn Bands, Soul Bands, Black Gospel, the groups that may use the Flat Keys more often than not, I take advantage of what I learned from transcribing Jimi and go ahead and tune the whole guitar down one half step.
That makes my "open E" fingerings into Eb, A into Ab, D into Db, etc. -- and makes the guitar playing as simple as playing in the "guitar" keys.
It also imparts a rather different sound, as well, often purty nice.
As Jimi, and later on, Stevie Ray, showed us.
--Mac