Related to closed ticket ANW-318... but it looks to me that if I want a MARC export to include a link back to the full finding aid, I have to manually enter it in the "EAD Location." If the ArchivesSpace Public User Interface is enabled, the MARC export should automatically include a link in the 856 field to the resource in the PUI.
This would probably need to be implemented as a configuration, as the PUI can be enabled for internal use without any intention of having other people see it. There's currently nothing that distinguishes internal use vs. external use of the PUI.