Rockbox 2.3 has now been modified for both the Player and Recorder thanks to devro.
To get up and running quickly, use one of the pre-built binaries from the archive.
ajbrec.ajz is for the Recorder (v1)
archos.mod is for the Player/Studio
Attach:rockbox-2.3-archos-2004-11-18.zip
It's quite likely you will want to compile it yourself to tweak the button mappings, or to build for another Archos model. Look in the file firmware/drivers/unilink.c for button customizations.
To apply the patch, use
cat <patch-file> | patch -p1
at the root level of the source tree.