If you haven't found it yet the DD-WRT site has a good guide to debricking the WRT54 series. »www.dd-wrt.ca/wiki/index ··· ad_Flash
If you can still ping it there is a good chance that you can use the TFTP tool (linksys' own) to try and reflashing it. If that doesn't work the JTAG method is your only option, on the plus side that model of router has the holes for the JTAG header, though Linksys for the reasons of cost control (ie make it as cheap as we can) didn't add the header. Still its much easier to add the header than trying to solder onto SMC leads or PCB tracts.
You know how to solder, its just a mater of making the cable and giving it a try.