Hi!
I've added more endpoint to SPARQL federation whitelist, including attribution-licensed ones. Please see the full list in the User Manual[1]. All endpoints are acknowledged at WDQS Licensing page[2].
You are welcome to nominate new endpoints at Wikidata SPARQL federation input[3] page. I've made some cleanups and edits there to make nominating easier, hopefully that helps. Examples and links to any materials showing how to integrate data between endpoints are very welcome too.
[1] https://www.mediawiki.org/wiki/Wikidata_query_service/User_Manual#Federation [2] https://query.wikidata.org/copyright.html [3] https://www.wikidata.org/wiki/Wikidata:SPARQL_federation_input
Thanks,