nixpkgs patches Ardour to scan for plugins in several additional places—
-
/run/current-system/sw/lib/{ladspa,lxvst,vst3}(environment.systemPackages) -
/etc/profiles/per-user/$USER/lib/{ladspa,lxvst,vst3}(users.users.<name>.packages) -
~/.nix-profile/lib/{ladspa,lxvst,vst3}(home.packages) ~/.{ladspa,lxvst,vst3}
—so it looks like just adding a package via one of those options and re-scanning in Ardour should work.