#355 (comment) mentions that you can use $NIX_BUILD_SHELL to specify the shell being used.

This does not work for zsh (and probably any non-bash shell) however:

% NIX_BUILD_SHELL=zsh nix-shell '<nixpkgs>' -A hello
zsh: no such option: rcfile

Source:

    $ENV{BASH_ENV} = $rcfile;
    my @args = ($ENV{NIX_BUILD_SHELL} // "bash");
    push @args, "--rcfile" if $interactive;

In #113 / ed09d0c, it was changed to use "bash" instead of $SHELL (by default).

Is it possible and worthwhile to add support for non-Bash shells?

This would also include patches to scripts like https://github.com/NixOS/nix/blob/master/dev-shell then probably.