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_…
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