boot into sigle user from cd

[Eric]

I am building a custom recovery cd and would like to have it boot into 
sigle user mode automatically.

Within the loader.rc file I have:
  set boot_single

which boots into signle usr, however, it prompts for the default shell;

 "Enter full pathname of shell or RETURN for /bin/sh:"

Is there a way to set the default shell so that it does not prompt?

Thanks
Eric