This tool will locate the floppy driver (DRVR resource ID 4) in a mac ROM and overwrite it with a ROMDisk driver, then recalculate and update the ROM's checksum.  It operates directly on the file passed to it, so you'll probably want to make a backup of the ROM before attempting this.

To build the tool, grab installromdriver.c, and run:
gcc -o installromdriver installromdriver.c

Alternatively, grab both the .c file above and the Makefile, and run:
make

This will give you the binary: installromdriver
There are two ways to invoke installromdriver:
./installromdriver <myrom>
This will install the romdisk driver in place of the floppy driver in your ROM and update the checksum.  It does not install any disk image into the ROM.

The second way is:
./installromdriver <myrom> <diskimage>
This will install the romdisk driver in place of the floppy driver, update the checksum, insert any necessary padding between the end of the rom and the 512kb location the romdisk expects the disk image to live at, and then copies the romdisk into the right location.  This should result in a ROM that is ready to burn.

Good luck, and remember to backup everything before running this tool.