As far as I know, Indigo doesn’t have a proxy system linking to external files. Therefore, I can’t create proxies for it.
Indigo only has a sort of proxy system that works only inside the model (a simple component linked to a more complex component), but with Skatter’s “Render only” feature, it brings no benefit to have the Library items using it.
…indeed Indigo DOES support external references for models (since a couple years, almost). Yes, it has the internal reference dummy system you’re telling me, but it ALSO supports xrefs! I use this feature all the time!
At the moment, it can xref to .obj and 3ds files: I usually use objs. You have the complex “tree.obj” anywhere in your pc, you model a simple dummy inside sketchup, make it a component, enter the component, right click and choose “link to external model…” and choose “tree.obj”. You can then spread the simple model inside sketchup and you have thousands of trees with null system usage.
Does it sound a useful piece of information to you?