Hacking away at Geektool I decided to try something else: sending raw terminal escape-codes at it and see how it handled it… In short: even better than I thought. So here is an update to my earlier Geeklet to display a calendar:
printf "$( cal| sed "s/^/ /; s/$/ /; /^ Su.*/d; \ /.*$(date +'%B %Y').*/d; /^$/d; \ s/$(date +%e) /\\\033[1;37m$(date +%e)\\\033[0m /g;" )"
I will admit to still using the previous version, though, since I prefer the font bolding in that one.