qa-page-admin-widgets.php file contains an array named: $templatelangkeys that is instantiated on line 72 (in my source).
$templatelangkeys contains a list of all avaliable pages (or "template" as they're referred to) that is displayed on the Admin -> Layout -> Add Widget page when selecting what template your widget is permitted to appear on.
You also must allow the widget plugin to display on this template through the widget's allow_template() method. (as you have noticed)
I haven't yet found a method to register a new template that references the custom page plugin (and it doesn't appear to do so automativally). If a method does not exist (I hven't found one) then you will need to hand edit the $templatelangkeys array when you want a custom widget plugin on a custom page.