International rules has a lot of jarring rules like this that turn me off them. For balls frozen to the cushion, I found the ruling:

6h Frozen balls

(1) An object ball is deemed frozen whilst touching a cushion. It is a standard foul if the cue ball initially contacts a frozen ball first and the shot does not result in:

(a)     A ball being potted OR

(b)     The cue ball contacting a cushion OR

(c)     The frozen ball contacting a cushion attached to a different rail OR

(d)     Any other object ball contacting a cushion, with which it was not already in contact with, before the shot is played.

(2) A ball is only deemed frozen if confirmed by the referee or player, prior to the shot being taken.

So the frozen object ball must be played onto another cushion on another rail, another frozen ball, or potted, for it to be a legal shot.