Very interesting! I don’t have a prostate so can’t comment on that side of things but your post got me thinking about some large anal beads that I haven’t used in ages. I’m wondering if something similar might give the sensation that you are wanting without the need for a trip to the loo! These are different to the ones I had but mine were discontinued and this is the closest I can find:
They feel amazing on the way in, especially with the large gap between the beads as they move up. I only used one or 2 at a time but they feel amazing. I’m thinking that pushing / pulling them out might give the feeling that you’re after. They do a bigger size too if you’re really brave!