Ok, so no bites, not totally unexpected. I've had this project in my head for about a year since I started modding my hummer, here are my idea goals
hummer board packet modem/tnc board 2 meter FM handie talkie board ps-one lcd screen and speakers dtvkernel/lunix kernel HT display and keypad one of the daughters cheap plastic laptops mic/headphone/ptt/antenna inputs/outputs
objective is an all in one packet/ham rig for things like field day etc im a kit builder/minor code chages to make things work (ie - no orginal thoughts)
most of the c64 modems/tnc plug into the cassette port which is not going to work the others are rs232 which it seems has not been added to the dtv/hummers
I remember building one for a friend on an original C64. The interface to the HAMRADIO was from the user port. We even burnt the software into a cartridge - therefore no need of 1541 drive. Of course, this is an interface to the HAM while you want to do it with the modem.
As for the hummer, GMoon has a wonderful project expanding the User port. It is somewhere on a thread of its own..it was even featured in either Makezine or Hackaday. Look into the circuit..you might be able to interface it
MadDog has also a good RS232 interfacing thread here at PETSCII.