Are such settings overridable in the runtime in case of trusted Postscript file source (as in: verified MetaPost script) or reasonable isolation measures (as in: container-like isolation with no network access and no write access outside tmpfs)?

The policy is in $out/etc/ImageMagick-6/policy.xml. Not sure how to override this at runtime.
Sandboxing ghostscript sounds like a better solution. That would catch all other programs accessing gs too.

Well, I normally sandbox the entire program anyway, because who can trust a PDF viewer to be correct on files not created in a control way. Sandboxing just Ghostscript sounds like a nice plan, but it does seem that, for example, GIMP uses libgs.so, not the command-line gs.

for example, GIMP uses libgs.so

OK, this makes it a bigger problem.
The quick work around for imagemagick could also be generating two derivations. One would then be called *-trusted and has the policy patch disabled.

As far as I understand it, the only imagemagick that includes ghostscript is imagemagickBig, and the only difference from the default version is the inclusion of Ghostscript.

Maybe the correct solution for Nixpkgs is to mark ghostscript vulnerable?

Should we just close this PR in favor of a more complete "ghostscript sandbox" solution?

Not sure about feasibility of Ghostscript sandboxing with libgs, but I do think that with Nix dependency model the change in this PR is not very useful in practice.

Closing in favor of #46047