Singlespeeding is definitely addictive, at least for me, but I can't say that I've ever done a long, loaded trip on mine.  There is something very organic about just having the one gear and the simplicity.  I find myself grabbing it more than any other bike I have. You'll surprise yourself at what you can climb on one.  If you're riding a 29er singlespeed loaded up for touring on anything but mellow rolling terrain, I wouldn't go much taller than a 32x21 or 22.  As far as rigid vs. suspended, I've only ridden rigid SS, but have had some trouble with tendonitis in my elbows from the shock and grabbing the bar tight on rough terrain (colorado front range) after repeated days riding my Niner.   I've heard this from other rigid riders as well.  And once you have it, man is it hard to get rid of.  Have fun....

Ride On,
PJ