I don't see why you can't do this in linux, it's just probably easier in freebsd.
You can take the 1.3 image, open it, and replace the files with the patched ones, then close it
There is a bash script that shows the steps in freebsd to do this
http://chrisbuechler.com/m0n0wall/downloads/workon.shit pauses with a prompt in the filesystem of the image, you can copy the newer files from svn in then exit the prompt and it rebuilds the image.
I have made a new image with all the current fixes including ayiya support and 1 or 2 experimental features here, it also contains sshd and tcpdump, so might not be to your liking.
generic-pc is tested, but embedded isn't. Feedback welcome
http://code.google.com/p/m0n0mods/downloads/list