Been there, got the scars to prove it!
I think the real answer is a "carb adjusting tool" like
http://www.probike.co.uk/it060008.htm
I've managed to do it with 1/4" flexible drives and long extensions, you have to hold the end of the flexi that goes into the carb to align the flat screwdriver bit with the screw, hence the scars (damn it gets hot in there!
), but it's not something I'd suggest everyone tries to do.
The rear carb is a little easier than the front, the long drive can go up from near the water pump, but even then it's a b@st@rd of a job. A very delicate touch is required and lots of patience. Do lots of dry-runs with the engine cold till you're satisfied you can feel your way round in there. After perfecting the technique, apply for your licence to become a gynaecologist!
A useful tip is to radius the end of the flat screwdriver bit, this means it doesn't need to be perfectly in-line, though obviously there's a risk of damaging the slot if you're not careful (though less risk than if it's flat across like a standard screwdriver and it's in at an angle).