Thanks to numerous contributions and a nice helping of determination (Bisq is a Java application after all), there is now a working Nix package for Bisq!
I still need to refactor the Nix code, but if you want to use Bisq on NixOS I encourage you to try the package and provide feedback. For what it’s worth, I successfully completed a transaction, and I’m confident enough in the package to have started another.
I’ve noticed that while the NUR packages used to work flawlessly now it fails to install with:
error: anonymous function at /nix/store/0v3jj4yywir5jn2a8gmbyl4jg9zd790l-source/pkgs/applications/blockchains/bisq-desktop/default.nix:1:1 called without required argument 'copyDesktopItems', at /nix/store/vffdfp5gnspq3mdid9wk5p15hmv3ci4z-20.09.tar.gz/lib/customisation.nix:69:16
(use '--show-trace' to show detailed location information)