After talking with @feijoas, there is a pull request for UxPlay in nixpkgs, however this does not work for me.
If I start uxplay -d I get gstreamer errors.
(uxplay:132635): GLib-GObject-WARNING **: 13:17:45.031: invalid cast from 'GstQueue' to 'GstBin'
(uxplay:132635): GStreamer-CRITICAL **: 13:17:45.033: gst_bin_get_by_name: assertion 'GST_IS_BIN (bin)' failed
However, if I drop into a nix shell
nix-shell -p gst_all_1.gstreamer gst_all_1.gst-plugins-base gst_all_1.gst-plugins-good gst_all_1.gst-plugins-ugly gst_all_1.gst-plugins-bad
and then run uxplay -d everything works.
What is the problem with the gstreamer plugins? They seem to be in the buildInputs. It does also not help if I add them to my configuration.nix. It works only from within the nix-shell.