Enabling Scintilla multiple selections is easy, but everybody seems to have missed the following:
Geany and plugins are __not__ written to handle multiple selects, its is unknown what will happen in Geany or plugin functions if there is a multiple selection. In the best case it will "just work" on the main selection and ignore the rest, in the worse case who knows, crash maybe.
Geany handles rectangular selections, but they are provided by Scintilla as a single selection, not multiple selections.
No plugin can change this as its in Geany and other plugins code.
Until the plan for introducing multiple selects addresses this it is likely to go nowhere.