There's a trick which I've found somewhere on this forum:
Screw the front of the pannier rail to the frame, but don't tighten it.
use a tiewrap (?) and lead it through the nut. Then lead the tiewrap through the mounting hole and pull then nut against the frame. Now use your thinnest finger to hold the nut into position against the frame while retrieving the tiewrap. Then bring the rear of the pannier frame into position (still holding the nut in position) and fix it with the bold. It's still not easy to reach the nut with an angled socket driver (?) but can be done... and you don't need to disassemble the rear.
Ah... it was Colros who came with that trick:
http://www.xt660.com/showpost.php?p=114061&postcount=10
HJ