algorithmic modeling for Rhino
I guess the clean solution is to define two different paths to reference the grasshopper libs for R5 and R6 (don't know how to do this or if it's possible). Otherwise I once compiled a python component with R6 (.ghpy), which is read by R6 only.
It would be good if somebody could show where to reference those paths.
You can get the default Grasshopper folders from the Grasshopper.Folders namespace. Not sure if that helps here, but like say this to reference the default libraries folder:
If you go to %appdata%\Grasshopper\ you'll find a folder called 6. Put Rhino6 only stuff in there.
I thought I created one automatically whenever GH starts in Rhino6. You can try adding a folder (can't hurt certainly). But it makes me suspicious. I'll have to talk with Will Pearson to see if we decided to change this approach.
The folder structure you'll need is %appdata%\Grasshopper\6\Libraries\ assuming it all still works.