I'm probably just being impatient. I'll do the local build thing with pacman for now, but my curiosity is pushing me to consider the orphan request option. I'll give it a couple more weeks though.
Regarding your question, you can just clone the package's git URL or download the PKGBUILD file directly, make your edit and run makepkg or makepkg -sirc as the wiki suggests to produce the package and install it.
You can also install the package tar with pacman -U <file>.
But looking at the comments it seems you are using an AUR helper that has a cache you might want to clear as the git repo for that package has an unstaged change for the license file for some reason (or you reset that file so git doesn't complain when pulling).
Did you change something in the license file yourself? Check your yay cache and perhaps delete the cached files for this package.
While there seem to be other issues with the PKGBUILD (typo for example), this seems like a problem with your local setup and not the package. Note that using AUR helpers like yay are not supported, so check if the package builds correctly manually.
Aside from downloading and changing PKGBUILD directly, there is an AUR helper named Paru that shows the PKGBUILD contents before using it to build the package. I'm not sure, but seems like it opens inside a terminal text editor, so you can make changes before building.