@florianjacob I'd appreciate your input on the interface here; if you have an idea for how to do it more cleanly, I'll be happy to update it!

Thanks for looking at this case, I really thought you could specify absolute paths to ppd files for model and are done with it. 👍

In general, I wonder how complex it currently is to package custom ppd files and whether that should be the One True Way™ to deal with custom ppd files.
In theory, it shouldn't be more than a simple derivation with a folder share/cups/model/<manufacturer>/<ppdfile>.ppd, adding that to services.printing.drivers = [ pkgs.gutenprint pkgs.foomatic-filters pkgs.hplip custom-derivation ]; and then one should be able to use model = "<manufacturer>/<ppdfile>.ppd"; - here model actually is a file path, but a relative one to share/cups/model. This is also the case for the hplip drivers, which also just copy their ppd files and don't use some form of drv:// protocol prefix.

As I never required a ppd file that wasn't yet packaged in pkgs/misc/drivers/ / pkgs/misc/cups/drivers, @ryantrinkle what do you think on path to ppd file vs. custom derivation? I would also imagine that if we aren't careful, the path to ppd file could end up more brittle on rebuilds when the file isn't present / its copy in /nix/store gets garbage-collected.

Concretely, I also think that types.either types.path or types.str; would lead to a better user experience, probably with an assert that in case type is types.str, it does not start with "/". Even if the options are then less aligned with lpadmin, but we're building a wrapper anyway.

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@florianjacob Sorry for the very long delay! I've tried out the approach you suggested, and it worked just fine - I think it's a better user experience, so I don't think my patch is necessary. Thanks for the advice!