I haven't looked at your code, but judging from what you've said, a couple things come to mind:
1) You shouldn't need or want to edit object_terrain in any way, unless it's to force a new interaction on it using an override (and if that's your goal, you can use scripts, which'll have better forward compatibility, since they can inject the interaction to object_terrain). I can share an example of such a script if need be. As far as terrain being recognized as a toilet, that's probably going to be an involved process and may not work for generic terrain itself; I mean, if you're wanting some kind of remnant of it happening on the terrain, I can see why you'd go in that direction, but you could probably create a puddle at the end of the interaction, which more or less simulates a result of the terrain being peed on. If all you want is for the sim to autonomously consider terrain as a possible option for peeing, you should be able to pull that off in your custom pee-while-standing interaction itself, by implementing some autonomy pull for bladder.
2) It sounds like you're on the right track with creating an interaction based on the pee while standing interaction. The route failure part probably has something to do with the animation and/or the constraints of the interaction. You could try experimenting with the removal of constraints, changing of animation, etc., to try to nail down where the problem area is.
