Hi.

Is an engine seized from timing belt break usable for a complete teardown and rebuild, or is it a few hundred pounds of scrap metal?

 

Well. that would all depend. If you rip the head off, check how the head is, any nicks? How are the gaskets? If you put some fresh winter oil (because it is thin) into the camshaft lobes, can you slowly start to free wheel turn the head? If you pour some oil into the pistons and put new oil into your oilpan, can you slowly freewheel the block with pistons (head still off) if both can be turned, Good to go, if they do not, you might have to start to take parts out and replace them

 

I have no idea what "tony" is talking about but If your engine siezed from the timing belt breaking it most likely can be salvaged.
The valves are probably bent (some of them) and possibly a piston or two has marks on it. It can be fixed, but will cost some money...

I recently bought a 93 hatch with a gs-r motor in it for 2k. Why so cheap? well pretty much the same thing happend, he missed a shift, tagged a piston and betnt a few valves. More money then the guy wanted to spend on the car so I got it for what the car was worth....I planed to tear the engine down (as I did) and build it up for boost anyways, so it was An awesome deal for me...
New pistons (in budget)
New rods (in budget but not needed to return to stock)
new head (he bought)
misc parts I bought.

All in all I got a motor to build (as planned in the budget) for a great price