Conditional loading based on the user's source files?
 

I'm currently writing a user file that's going to contain some fixes(I'm trying to make narco work, among other things) and houserules. So far I've been using the "Replaces Thing" functionality quite heavily. It's the best way I've found of letting people with existing save files use modded items without having to replace all of their gear and qualities and skills.

The problem I'm having right now is that some of my houserules and fixes change behavior of things in optional source files, and not everyone using the mod has every Shadowrun package installed.

Is there a way to have a user file conditionally replace things, or some technique that I can use to make things work?