I'm trying to convert a Trek 5200 frame into a cross bike...so I'm curious if anybody has any experience with rear tire clearance. I'm looking at tubulars tires and am asking if anyone has used the 28mm Tufo or Challenge tubulars on a road frame. Or are their other Good tires that work?

this is probly a bad idea, not enough clearence and the geometry is wrong

This was done in 'the old days' when bike's where made out of steal and had lots of clearence and quite big tires anyway.
I very much doubt that you can even squeze a 28mm road tire in your trek, let anone a cx-tire with knobbies. Only thing would be to try, but don't count on it to work very well if it fits + 28mm tires aren't very nice to ride on a cx bike, these are only for specific races.
Try and sell your frame and buy a low end cx frame instead would be my advice if you are serious about riding cx.

I've gotten CERTAIN 28's on a road bike. The Tufo's are tall.

No matter what you run, the clearence will be super tight at the top, and you will have a lot of mud clogging it up.

You'd be better off getting a frameset to swap parts over to; either cross, or hardtail MTB w/ 700's and some spacers for the rear wheel.

The vittoria cross tires in I think 28mm will fit most road bikes. I train in the winter with them on my road bike. The better the bike, usually the tighter the clearance and they may not fit on high end road bikes.

i used clement grifo(diamant profile)for paris roubaix, they are very small(around 26 mm).i have several grifo neve(the profile of the old dugast and gommitalia), they are also thin. but the profile will take up a lot of mud, and this will be in the brakes/frame while riding.
it works to train in the winter with grifo tubulars, but i would not try to really cyclocross with a road frame.