No I don’t use that module as I have always used particular applications in particular workspaces so my muscle memory pretty much has me covered. I see in the wiki that the workspaces module does require special attention, so I checked their flake, and indeed they export a special package for it:
❯ n repl github:hyprwm/Hyprland/v0.24.1
Welcome to Nix 2.13.4. Type :? for help.
Loading installable 'github:hyprwm/Hyprland/v0.24.1#'...
Added 7 variables.
nix-repl> packages.x86_64-linux.waybar-hyprland
«derivation /nix/store/7a0skd68bkzvfym4l3b6hwar9jn8y7km-waybar-0.9.17.drv»
So just grab the package from the hyprland flake, instead of from nixpkgs and that should do it.