After some digging, I found related bugs #31309 and #33566, the latter which
introduced the current `:format' as a workaround to an error.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=31309
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=33566

From what I gather: a colon is necessary, for whatever reason.  So having a
tag-less type, as `group' claims to be, is not possible through normal means.

What *is* possible, however, is hiding the colon with a display text property
(The following should go in `group's `define-widget' form, at wid-edit.el):

    :format (concat (propertize ":" 'display "") "%v")