But since the use case is personal dotfiles, I imagine the user isn't going to forget that they set this up.

Generally, good behaved applications have an entry in their man page that spells out these details for you, so you don't have to work out anything.

Also, completely unrelated to my motivation, someone pointed out that modetc could be used to quickly hotfix packages built with Nix. Say that you need to fix a CVE in openssl, normally that would require to rebuild all dependent packages, which takes a long time. Instead with something like modetc you could build just openssl and rewrite /nix/store/<hash>-openssl-3.6.0/ -> /nix/store/<hash>-openssl-3.6.0-hotfix/.

Another application might be replacing some configuration file with placeholders for secrets, with one file with the secrets substituted in, without having to modify it in place, possibly only for a specific UID. This is something you might find useful on NixOS.