"I wonder if the theory could be extended to guitars - those folkies who couldn't play in, say Ab, could extend the neck length by 4 frets and play in C... More fun than capo'ing on the first and playing in G - and think of the tone you'd get!

I've been thinking about buying a Baritone acoustic guitar for a very similar reason!

A reverse capo: there are certain songs I've learned to play and sing a certain way which, on reflection, having recorded myself singing them, I would prefer to pitch a little lower.

I'm not sure what the standard tuning for baritone guitars is supposed to be. There seem to be more of them in the US than the UK. (at least, acoustic ones, anyway)