I just did some wandering around on crazyguyonabike to see if I could find any folks mixing mountain biking and touring. You might want to check out Christian Bomio's journal http://azoNZo.crazyguyonabike.com titled azoNZo - Mountain Biking New Zealand. It looks like much of his serious mountain biking was day rides, as in without carrying gear - but he also used his bike as his means of transportation. It might be worth signing his guestbook to ask him if he mixed single-track and touring (as in did the single-track with the panniers on his bike). Click here for a view of his loaded bike.