(I hope I don't screw this up).
A slash rated breaker would say 120/240. If this is on the breaker, then the voltage of any ungrounded phase must not exceed the lowest rating when measured to ground and the voltage between any ungrouded phases must not exceed the higher rating.
So, if you used this breaker on a high-leg phase and another phase, you would meet the 2nd requirement because the voltage between phases wold be 240-volts. You would not meet the 1st requirement because the high-leg voltage to ground would be in excess of 120-volt (lower slash rating of the breaker).
To use the high-leg and another phase, you would need a breaker that did not have a "/" rating. As Augie pointed out, the cost of these breakers is quite a premium.
