A rule that conflicts with the Constitution is void.
I do not feel that this particular rule is unconstitutional.
The Constitution states, "Each House may determine the Rules of its Proceedings." Of course, this entails the power to determine the length of debate. If the Senate determines that debate on some topic shall continue until a two-thirds majority votes for cloture, then it is well within its powers.
That said, if the Republicans do pursue the "nuclear option," no court may reverse their actions. The Senate is the ultimate judge of its own rules; no judicial body may intervene.