It should be possible to show a list of "pages transcluding this page" from the Page namespace as a dropdown in the tab-bar
Various Wikisources have gadgets or scripts for this:
- enWS: https://en.wikisource.org/wiki/User:Inductiveload/jump_to_file (amongst other things)
- itWS: https://it.wikisource.org/wiki/MediaWiki:Gadget-puntanoqui.js