Create a spam key, the same as a Click Castsequence.

The first button called, calls a castsequence.
No commas for timing, just a straight looping sequence.

The second button is a simple /Cast Whatever.
Just a single spell.

Currently, the second spell casts whenever it is available.
And when it is not, the castsequence endlessly loops, as expected.

So, in his example, Ret's will endlessly loop:
Crusader Strike > Judgement > Crusader Strike > Divine Storm
But they will also: Exorcism whenever it is available, without needing to manage the proc at all.