Just FYI.
As ETA for this work:
https://phabricator.wikimedia.org/T128173
cannot be even estimated, we have utilized in pl.wikisource the old Swedish solution (by Innocent bystander) for providing interwikis based on the work/edition Wikidata model.
This is a modified sv.ws solution with an anti-loop protection. This solution also (unlike the original) merges iw links from the "work" item if there is no "edition" item from the specific wiki. (Such pages seem to be "old" interwikis in most cases.)
Of course, this sulution works only if there are already appropriate data in Wikidata; and moving the data there seems to be the main problem.
The pages utilizing this feature can be found in the category:
https://pl.wikisource.org/wiki/Kategoria:Strony_z_interwiki_utworzonymi_za_p...
New feature is visible in works of: Byron, Lagerlöf, Moliere, Shakespeare, Sienkiewicz, Verne and of few others.
If anybody interested, I can provide more technical details.
Regards Ankry