@cks I played around with xfontsel(1) but it doesn't seem to let you put arbitrary text. Then fiddled with xmessage(1) but it doesn't seem to take font-resources.
Here's an uuuuuuuuuugly (and there aren't enough letters there to express the extent of the ugliness) hack, but it seems to work for me:
$ xclock -digital -strftime "This doesn't need to be an actual strtime format string" -face Sans-24