An obvious question is how is the cluster going to be played, if the organist is using both hands for something else. Of course, you can use weights to hold down the keys, provided the organist has time to manipulate them. It's not entirely unknown for an assistant (whose main functions are to change the stops if the instrument doesn't have modern automation technology, and to turn the pages) to play a few notes when required!
But aside from that, it's quite common to use more than 3 staves for organ music that requires complicated registration. Split up the notation so that the music for each manual is on one (or even two) staves of its own.