Adding this would seem like a low effort/high reward type of thing. It would certainly open up a lot of possibilities in seeing how the Trilogy is used as a whole, instead of analyzing individual sites in a vacuum.
To make it easy to integrate into the current structure, this data could be exported in a new "table" with 4 columns (one for each Trilogy site user id; NULLable to account for no linkage), with one row per user.
Obviously, this would be a Trilogy-specific feature, so in terms of file structure, the new data should go in a separate folder from the 4 dump folders.
Since everyone's linked accounts are shown publicly on the user page, I don't think there are any privacy concerns that need to be dealt with here.