What would you have done?

[Fore Rail]

I think this is a CSI-only rule. Note that in CSI, just putting your hand in the pocket at the wrong time is itself a foul whether you touch a ball or not:

Placing hand in pocket: It is a deliberate foul if you catch any ball that is falling into a pocket, or place your hand into a pocket while any ball in play is in motion near that pocket. (AR p. 97)​

Going back to the OP's question about a player putting his jump butt in the pocket. If he would have placed his hand in the pocket to retrieve his jump butt while any ball was still in motion near that pocket - foul!!

Imo it wouldn't be a bad idea for every organization to adopt this rule.

 

[MattPoland]

And please note that the BCAPL/CSI rule only applies in their events. Under WPA/WSR the shooter is responsible for any extraneous contact, such as with a template that their opponent has removed from the rack area and placed overhanging the nose of the cushion.

I thought of a related situation...

Suppose I know that a pocket rejects balls shot hard and I have to shoot hard. Am I allowed to place a wadded up paper towel in the bottom of the pocket to make it work right? I recently tried to do a lesson on a table with insanely sticky pocket liners. Any ball going straight against the pocket liner with good speed and roll got rejected back onto the table. Could I hang a paper towel over the back of the pocket to reduce the rejections?

The rules don't cover all situations. I think the general concept to apply is that you are allowed to take steps to make problem equipment function reasonably, so that fair play is possible.

CSI Rules have a few remedies for that. I assume if no ref is present then both players would have to agree to any modifications.