@DarlesChickens I'm not an actually modder (so it what i'm saying might be wrong and I can't actually make this into a mod), but based on my experience on how ksp part modules work i think a alternate and easier to implement solution (since stock ksp visual effects aren't created to run on waterfall and the mod might not support effects on non-ksp part instances so it might be quite challenging to implement it the way you described): making a small part with a extremely efficient jet engine part module configured that produces no thrust and only works at high speeds and high altitudes inside atmosphere of all celestial bodies (i.e. use intakeAtm instead of intakeAir) that will show combustion failure at all other non-reentry conditions (i'm pretty sure this can be done). Then, you can create a upside down waterfall plume for the 'engine' to use that looks like an actual reentry plasma trail so when you place this reentry effect generating part on the bottom of your vessel, all you need to do is to activate the 'engine' and when the reentry conditions are met, it should produce a stunning plasma trail. Obviously a disadvantage would be the fact that you need to place this part on every ship you build and will no longer count as fully stock (if you usually don't use mods), but it should be relatively easy to be made into a mod for players to use conveniently since this part would technically be an engine and waterfall is designed to make plumes and effects for engines. This is similar to the idea of a mod I have seen (probably vapor vent or plume party) which has a supersonic visual effect (but without waterfall) that is activated when that part is placed on a plane which is going at mach 1.
Hope this helps.