I was at a session today where I had been planning to play one song, in aEAC#E tuning (w/o capo.) Then they began to play a tune in G which I wanted to join in on. I didn't have time to retune, and I wouldn't have wanted to go down to G and right back up to A without a good reason.
I had a capo handy, but obviously I couldn't "capo down". So, what I did was to capo at the 3rd fret and play out of the D major fingerings from Open G tuning, but actually sounding in G major. I wouldn't usually use Open G for playing in D, but it's fine for a lot of tunes (and I like Standard tuning on the guitar for D well enough.)
This put me in aGCEG (the 5th didn't matter, since I was picking not clawing.)
Although the tune was quite short I feel that this tuning worked well. I realize it's the same tuning as a pony banjo, and an octave above some minstrel banjo tunings. It's a useful way to get into something resembling a G tuning if you're pitched above G, don't want to retune, and have a capo handy.
I may have to explore the tuning more at some point. Obviously for backup I could just play G chords out of A tuning, but since I wanted to have a stab at picking the jam tune on my open strings (and not have to mute the C#), I felt it was a fruitful experiment.