Just got this frame to build up to into a single speed cyclocross. Problem is that it doesn't have canti bosses. My question: Is there a way to put cantis on a Miyata or any frame without canti bosses?

No, although you could swap out the fork for one with bosses.

Brazing canti bosses on at your local framebuilders should be in the neighborhood of $70. Before you go to the trouble, make sure you can fit the tires you want.

From the photo I found, looks like the calipers it came with would give you enough clearance. Yeah sure, mudshedding blah blah. Just get some new cables, housings, and pads, clean and lub the calipers, and Bob's your uncle. I'm curious to see the final result of your conversion.http://upload.wikimedia.org/wikipedia/en/thumb/8/84/Miyata710.jpg/800px-Miyata710.jpg

Brazing canti bosses on at your local framebuilders should be in the neighborhood of $70. Before you go to the trouble, make sure you can fit the tires you want.

Don't forget the cost of re-painting too.