It would be nice if jackcess provided a customizable LinkResolver utility which made it easy to load linked data from files which are not access databases. the LinkResolver api requires that the data be provided to Jackcess as Database/Table. You could create a temporary database, load the external info into it on demand, and provide that temp database as the secondary source to the primary db.
Inspired by this idea: http://stackoverflow.com/a/43748207/552759
Diff:
Added CustomLinkResolver which should provide the relevant functionality.
Fixed in trunk, will be in the 2.1.7 release.