@cmb:
Yes. I don't have the exact instructions at the moment. I'm going to finish this page as soon as time permits.
http://doc.pfsense.org/index.php?title=Modifying_Embedded
Or maybe Jim will see this and fill it in. :) I know he's done that before.
Hah. I didn't see this message, but I saw the wiki page. I almost filled it in but didn't since it looked like you were in the middle of it. I'll do it tomorrow, I have a batch of 7 shiny new ALIX boxes I need to image and I'll need to be doing some tinkering along these lines with them anyhow.