This is exactly what I meant with "there is one but it is slow and limited last time I tried", but it does seem that they made good progress (the last time I tried it was some months ago).
Time to try it again.
BTW, thanks @easygenes. It is really much better nowadays (that goes to show how fast Nix/NixOS is improving).
Thanks for following up, which makes your earlier comments more useful for folks wondering about the present state of things