No problem, as long as you don't use a 10s (Dynasis) MTB RD.

You can use just about any Shimano RD except the above, or a RD-7400 (8s Dura Ace). And possibly those belonging to groups using the aero STIs, which have a slightly different cable pull.

I don't know MTB stuff too well, but AFAIK XT is roughly equivalent in quality to 600/Ultegra.

Something like this in a medium cage would be pretty appropriate, I guess.

If you want to be fancy about it and your bike isn't a triple, you could probably swap the cage out for a short one off a road derailer, but you'd need to find one with the same cage mounting setup (there are 3 or 4 different styles).

Actually come to think of it, I have just such a RD here, from a still-born project:



You can have it for AU$30 + $20 shipping (Aus Post Registered Post International or Pack and Track International), which today comes in at just under US$45 total.

Missing a little paint as you can see, but it works a treat. I think Deore is equivalent to 105.