Summary
It should be able to use Page previews on pages in (and from) other namespaces than the main namespace.
Description
Extension:Popups can be used by other MediaWiki users (and perhaps within other Wikimedia projects) that could benefit from having a configurable option to allow content from other namespaces to be included in a popup.
This could be a site-wide configuration option - allowing wiki administrators to enable/disable on a set of namespaces.
It could also be user-configurable as a power-user setting for contributors (see NavPopups as an example) allowing for a more advanced interface to previewing wiki content across namespaces.
As discussed: https://www.mediawiki.org/wiki/Topic:Ufna3qrxehzhcud1
Developer notes
We already provide a certain level of configuration via $wgContentNamespaces.
The summary endpoints do not work on most namespaces and many namespaces would need special treatment, which would add a high amount of code complexity.
e.g.