I run Debian's Chromium package on a 32bit NixOS machine, since cache.nixos.org stopped providing non-essential builds for 32bit a few years ago and I building Chromium from source myself was too slow.
My solution was to fetch a Debian filesystem, chroot into it (using proot) and run 'apt-get install chromium'. The result is an immutable Debian with Chromium installed. My chromium executable is then a script which chroots into that filesystem (again using proot) and runs chromium.
It's not reproducible, but it's simpler than maintaining a list of all the required .deb files and their SHA256.
You sort of have that with NixOS as well, with eg. steam-run that provides a debian-like chroot and can run most executables. (You can also modify it to include any dependency you like, that will get installed to standard locations)
