I may be missing something here but why not use
install56.fs
with
# # dd if=/location/install56.fs of=/dev/rsd1c bs=1m? This assumes the drive is recognized as sd1.
See the
FAQ for more details.
After the install, reformat the usb drive
# newfs -t msdos sd1c
Download the firmware (assuming you have access to a second computer) and copy it to the thumb drive. Load the firmware on the new OpenBSD 5.6 install from the thumb drive, configure wireless and go from there.
What would be kinda of neat would be to mount the image loopback and insert the needed firmware. The Debian developers also share the OpenBSD view on distributing proprietary firmware and have an install option where the firmware can be accessed from removable media (floppy, usb thrumb drives and CD's). I wonder if it is possible to escape to a command prompt during the OpenBSD install and mount a usb drive and load the firmware for a network install?