@Watcher: do you mean to use the 32u4 as a bridge, i.e. the host on its USB can upload the image (hex file) through the 32u4 to a remote rs485 device (perhaps a 328p)?
If so the problem that I am aware of is control of the transceiver, which has a line that has to be controlled to talk, and another for listening (though it can always be on).
Since the upload starts with avrdude I would look there and see if someone has done a protocol that works with RS-485 and how avrdude controls that talk line. My guess is the avr109 protocol may have a way to control the transceiver, if so then have a look at xboot for the bootloader which I seem to recall has RS-485.