It has been explained in a number of locations that the Proofread page extension and LST currently do not have a means of consistently supporting 'sidenoted' or 'side-titled' material.
It would be helpful to identify what the current lack of functionality in Proofread Page prevents consistent display of side-titled and sidenoted content.
An examples of a sidenote/side-titled work is - https://en.wikisource.org/wiki/Railways_Act_1921 which is transcluded.
The DIV-based approach being used (on this work) is not a permanent solution, and in any event it's a bit cumbersome. I'm fully expecting it to break on a future upgrade, It's already incompatible with dynamic layouts
in so far as it makes some assumptions.
Another disadvantage is that the 'layout' information has to be re-entered for every single paragraph (and sidetitle) owing to what I've been told is variously limitations in the parser, in LST, in proofread page, or the outright complexity of the template itself.
The alternative span-based approach (used elsewhere) is also not a permanent solution, as sometimes with some works the sidenotes can overlap, leading to a messy rendering.
https://en.wikisource.org/wiki/Index:Historical_Works_of_Venerable_Bede_vol._2.djvu has examples of both the
DIVand SPAN based approaches to sidenotes.
Neither of these are a permanent solution.
Self-withdrawal on the grounds that implementation of side-titles support is a feature that was been mentioned as an issue elsewhere and this particular feature request hasn't been the subject of a full discussion on projects that may be affected.