I just found out that vpnc now seems to be part of the vpn build of the latest dd-wrt. Looks like I can finally try vpnc on my WRT54GL without too many difficulties. Although I don’t expect it to perform too well. The wiki page explaining vpnc on dd-wrt can be found here: http://www.dd-wrt.com/wiki/index.php/VPNC
Looking for that, I also stumbled upon the Firmware Modification Kit, which seems to easily allow the extraction, modification etc. of the firmware images.