Hide time signature numerator?

[Owain Sutton]

I'm putting together worksheets for children, where various pieces of notation
are missing, for them to fill in. One thing I'd like to do is hide the top half
of the time signature, but don't know how to do this.

The snippets repository comes close -
http://lsr.dsi.unimi.it/LSR/Search?q=timesignature - but my knowledge of
Lilypond is stretched by those examples, and so would be grateful for help!

(Apologies if anyone receives this twice, but it didn't seem to have got 
through earlier today.)