I'm getting into some trail riding, the bikes I currently have access to are a Canondale T800 and an old crappy too-large MTB (Shogun trailbreaker 1). I'm considering putting cyclocross or thin 29er tyres on the T800; only problem is there is some toe overlap which will get worse with the big tyres. I can get the handlebars high, the brakes are OK and the gearing is ideal.
Sure a decent MTB would be nice, but around here it takes $550 to $750 to get something adequate. I'm over the Shogun, except for practicing some skills in a train backlot nearby.
The other thing I might try is moving the 26" wheel / 1 1/4" headset crappy suspension fork onto the tourer, to see if toe overlap is any better.
I tried it with 26 x 1.75" tires on my tourer, it worked OK. The long wheelbase doesn't help, and the low bottom bracket doesn't help either, but I still had a kick. Dirt roads are fun, real easy singletrack is fun, technical single track is not such a good idea. The upright position of a mountain bike makes picking a line and keeping the bike upright on technical terrain much easier than a drop bar tourer.