>   Using characters other than * to indicate variant forms. I can
    >   imagine a command with two independent variants, accesed by
    >   \foo, \foo*, \foo+, and \foo*+

   this could be provided easily enough, eg, S{<symbol-to-scan} and
   might indeed be generally useful, any comments?

A very nice idea!  I like it.  But the \foo*+ form is too much -- a
single character should be sufficient.