Page: pages are not plain wikitext pages but are split into three textareas: header, footer and body and a proofreading level tag (note 1).
So, an adaptation of the VisualEditor to these pages is needed in order to make edition of the pages work.
The UI need to be tweaked in order to allow the splitting of the main textarea into three and the edition of the Proofreading level at the save of page as it works currently for wikitext editing (example: https://en.wikisource.org/w/index.php?title=Page:La_Fontaine_-_The_Original_Fables_Of,_1913.djvu/20&action=edit ).
An other useful (but less important thing) things is to add modules to the Visual Editor to support specific tags used by Wikisource like <pages>, <section> and <poem>.
note 1: structure of the page as save in the database:
<noinclude><pagequality level="LEVEL" user="LAST PROOFREADER" />HEADER<div class="pagetext">BODY<noinclude>FOOTER</noinclude>
This card tracks a proposal from the 2015 Community Wishlist Survey: https://meta.wikimedia.org/wiki/2015_Community_Wishlist_Survey
This proposal received 41 support votes, and was ranked #23 out of 107 proposals. https://meta.wikimedia.org/wiki/2015_Community_Wishlist_Survey/Wikisource#Visual_Editor_adapted_for_Wikisource