Since I have just received the following feedback from upstream concerning the addition of a PPD + patch to rastertolabel.c to CUPS:
Sorry, but we will not be adding another driver to the CUPS distribution at this time. Any future label printer driver support will come as part of the LPrint Printer Application.
and LPrint - A Label Printer Application not being available as a FreeBSD port yet, I'd like to ask if anyone is willing and able to have a look, as that would make it possible to use the most common label printers with FreeBSD.