have you looked around the ical package?
i just noticed that in ical:contents:resources there is a iCal-Empty.icns file. it is the iCal icon, but it does not have any date on it. and then in the English.lproj folder there is an iCal.icns file, which looks to be the default iCal icon with the 17 date on it.
i should not be sitting here, i should be studying right now, so this is why i am not testing this. but, try create an icon with the same number area dimensions and angle as the normal iCal icon and then call it iCal-Empty.icns and replace the resources icon with that and then put one with a number (does not have to have a number) in the english.lproj folder and then try run iCal.
then you can see if iCal just puts in the date regardless of the icon. if that is the case then you can take it a step further and start looking around the files in the package and see if anywhere there is a font and a size thingy that controls the text.
but hey i could be *way* off the mark, i only started using osX and a mac a few months ago.
good luck, and if you can/feel like it, post your findings and after finals (wednesday), if you have not gotten anywhere i will start messing around and seeing if i can get anywhere.