Can I use an officially released build of Caddy and have that official Caddy executable load FrankenPHP?

Caddy plug-ins are statically compiled into the server. Generally you need to build your own binaries if you’re using plugins. There’s an official script, xcaddy, to make that easy.

Hard to think this method takes off as people are reluctant to run their own binary that can't be updated with the rest of the system which in turn gives less users to report bugs and blogs about usage/reviews.

Depends on the package manager. Some package managers support overlays so you can keep your package modifications as a layer on top of the base package.

I know you can using Nix for sure. I think Arch/pacman (Arch Build System) and Gentoo/portage (since it is source-based) also have this concept, but I’ve never used it. No clue about other package managers.

The downside is that package updates will require a recompilation, but you can use binary caches to centralise this slightly.

Definitely! (using xcaddy and compiling PHP yourself, for now)