Aside from the frame not being cyclocross geometry, the brakes and tire clearance are really the only problem. Of course, this makes the frame & fork unusable in cyclocross. Cyclocross bikes use either cantilevers (preferably) or V-brakes. You could use everything on the bike except the frame and fork (sell on Ebay) and buy a cyclocross frameset and some cantilevers. Cantilevers work with road brakes.
The triple, though not ideal, can still be used. Standard cyclocross bikes use doubles. But then again go to a cyclocross race and you'll soon see that standards only exist in the new bike market. All manner of bikes are ridden at races. Many people even ride single speed cyclocross so they don't have to worry about derailleurs and mud. So to those people, even double cranksets are bad.
I'm going to do the same thing someday with a Trek 2000. Probably going to buy a Soma Double Cross frameset, but if I can find a lightweight steel touring frameset (cantivlever mounts and lots of tire clearance is standard on these frames), I'd go that way to save some coin. Sort of hard to find a good bike that is cheaper than a new frameset though, and the frame is all I need.
Interesting - I understand the points regarding the brakes and wheel clearance, but how is the geometry different from a road bike? Is the head tube longer or something to get you more upright?
Usually a higher BB and maybe slacker head angle.
Noone says you HAVE to use fat knobbies. Depending on the course, you may be able to use 700x28 tires with just a bit of tread. You'll want the knobbies and clearance for them (frame, fork, and brakes) when the courses turn bad.