What I ended up doing on SG site is to pass in source_package_id as a xinha_option in richtext-procs.tcl and use that in OacsFs.js so that its propagated up to the point of file-selector.tcl in xowiki. That in turn would allow to query for a parameter, say FSPackageID specific to each package as we have the source_package_id on where xinha is being called.
In file-selector.tcl, That has priority over all fs instances assuming none was passed. We are also able to add in more package parameters like in most of SG clients's cases, UseLRNFs or UseFOLIOFs if .lrn or dotfolio is installed respectively.
It's generic enough for Solution Grove's roster of clients but I don't know if it will be generic enough for everyone else. Just posting here so that one would know where to get the code if one needs the same functionality.