I have two spaces left in a three phase 240 volt panel, I want to install a single phase 240 volt breaker for a compressor. Is this OK if one of legs is the "high leg"?

Yes, as long as you are careful of any 120V loads or controls...
There is usually a small control transformer. If it is a 240V transformer you
Are OK, if it's a 120V then be careful which leg it connects to...