It is possible, there are a couple of very good guides on the Internet, it involves opening the router up and shorting 2 of the pins, then flashing the firmware over TFTP.
I bricked by WRT54GL twice in the same day, and managed to revive it using that guide. It's now running strong.
Just found the guide too, here:
http://voidmain.is-a-geek.net/redhat...g_revival.html