Hi,

project-switch-project currently only allows for single-char bindings to
be used for its commands.  Starting with emacs -Q, evaluate

    (require 'project)
    (define-key project-prefix-map (kbd "M-s g") 'rgrep)
    (add-to-list 'project-switch-commands '(rgrep "grep"))

and then run project-switch-project (by default bound to C-x p p) and
choose a project (hopefully you already have one available).  You will
see "[M-s g] grep" shown in the prompt, but you will not be able to
choose it by typing M-s g.

The patch attached bellow allows for any kind of key binding to be used.
Admittedly, there may be a better way to do it since I did not spend
much time looking into it.

Please, share your thoughts.

Best regards,
Dario