Recently discovered that RV waste gate valves probably make the ideal gunge tank valve. I say probably because the project I decided to go do after discovering this got really complicated but i'm close to testing a solution to interface it with a bucket. A company called "Valterra" makes them and they come with kits to actuate them with a cable handle. i've posted some photos on my profile of a setup i've been working on to open and close one pneumatically. They are pretty cool!

Correct - some of the tanks here in the UK use them as the release mechanism, and the quick and manual ability to release gunge over the subject sat beneath. I know the Great British Gunge Tank on Instagram and various socials utilises the 3" variant.

I've also got one in the 3" and 4" variants to play around with. Especially as a manual "quick pull" can open it fast to create the impactful drop that people want.