The GH_DocumentAssociations type exposes the following members.
Add an association between a Rhino and a Grasshopper file. If a similar association already exists, it will be overwritten. This function reads and writes to the disk, it is SLOW. Only call it when you KNOW something changed.
Find all the Grasshopper ghx files that have been associated with a given Rhino file.
Find all the Rhino 3dm files that have been associated with a given Grasshopper file.