There are several types of reversal emulsion that rely on different mechanisms. This was one area that I didn't get into much as they were pretty much history by the time I enetered emulsion work. They were used in the Kodak instant film primarily.
The most prominent relied on a core shell emulsion which trapped the negative image internally and used a nucleating agent to 'fog' the surface to yield a positive image. The nucleating agent was (IIRC) a hydrazine derivative called a hydrazide. Since I did my thesis on hydrazides and hydrazones as well as azo chemistry, I followed it pretty well, but have forgotten most of it.
I have some notes here somewhere if you are really interested, but I have not looked at them for years. If you are interested, I could look them up, but I can't promise how complete they might be. Sorry.
In any event, I have enough on my plate right now without getting into these rather more complex emulsions. I want to make 3 emulsion types and do them well and do them simply.
1. Contact emulsions in 3 grades. Pretty much done. They are equal to or better than AZO.
2. Enlarging emulsions in 3 grades. I have 2 out of 3, working on the next one. These are a cross between Brovira and Kodabromide.
3. Negative film emulsion with ortho sensitivity and about 25 - 100 speed. I'm getting 40 speed regularly with high fog and low contrast. I'm working on this.
4. Move on to color??????
Enough for me do you think?