Well, in the meantime, experimentation has yielded me this:
Code:
{tags:format_number()}
By passing nothing to format_number(), I I got the formatting to fail, thus passing null to the plugboard. It's totally a hack, but it seems to works, and doesn't seem likely to set fire to my computer, so that seems to be good.
I guess I could do the same with boolean testing for random garbage, or days_between() a non-date.