Extended mensuration signs in markup via Scheme

[Graham King]

I'm a novice Scheme programmer writing a music-function for manipulating temporal proportion in music transcribed from white mensural notation.  Amongst other things, the function includes the original mensural timesig in a \mark \markup, using the following code:

>     #(define mensurationSign '(("C"    . "timesig.neomensural44")
>                                ("cutC" . "timesig.neomensural22")
>                                %... and so on...
>     ))
>     
>     mensuration = #(define-music-function (P L mensuralTimesig %.....)
>     % ...
>         #{  \mark \markup {
>                 \concat {
>                     \musicglyph #(cdr (assoc mensuralTimesig mensurationSign))
>                     % ... and so on...

So far, so good - this all works fine.  However, I would like to extend this to work with mensuration signs that don't appear as musicglyphs.  For example: "C2" or "3".  Please could one of you experts point me in a sensible direction to achieve this?
(I could probably do this by splitting the mensuralTimesig argument into two separate arguments, but this would create an inelegant interface to the function.)

Thanks in anticipation of your sage advice.