Nevertheless, I find myself in a quandary. I would like to have a 64-bit version of a Notepad++ plugin (specifically “TLDatLexer” by Jason Newcomb), that has not been updated since May 2013. As far as I know, this plugin has never had a public x64 release at all, but I still want to build it for that architecture.

Could someone walk me through the process of building a Notepad++ plugin, preferably using the AppVeyor CI?

My fork of the plugin repository is here, in case it turns out to be relevant: [https://github.com/LordNyriox/TLDatLexer]