This holds no answers - only failure - until I have time to return to it later.
I'm trying to turn a WRT54G (I have five of them) into, essentially, a WiFi card. Technically, a "Repeater" (but not a "Bridged Repeater"). A "Bridged Repeater" is actually slightly harder to set up, but easier to get your head around.
Imagine you have your main wireless access point (WAP) called "homebase" that's handing out IP addresses in the 192.168.1.* range. If you have a corner of the house where the computers can't reach "homebase," you can set up a repeater that's closer. A "Bridged Repeater" will allow all requests to pass through it to "homebase," so computers in the far corner of the house will get IP addresses in the 192.168.1.* range, and all computers attached to the repeater will be visible to computers attached to "homebase." But if you set up a standard repeater, it will act as its own independent WAP - it attaches to "homebase" and uses it to get to the Internet, but it hands out its own IP addresses (perhaps in the 192.168.2.* range) and with this configuration all computers attached to the repeater are invisible to those attached to "homebase" (however, computers attached to the repeater can see computers attached to "homebase").
I followed the instructions at https://forum.openwrt.org/viewtopic.php?id=39077 , which my friend Daniel said worked very well for two different TP-Link routers. I modified four files in /etc/config/ : wireless, network, dhcp, and firewall. After a reboot, I have a WRT54G that looks happy, but which I can't connect to at all.
So what I need now is Failsafe mode (which varies from router to router: I'm talking about a WRT54G here). The correct method to get into failsafe mode is to unplug everything from the router, including the power, hold down the reset button on the back of the router, and plug the router in. Now watch the front lights: the "Power" light will come on and blink as expected: once the "DMZ" light turns on, release the reset button. Shortly, both the Power and DMZ lights will be blinking. This worked fine for my WRT54GS v.4, but doesn't work at all on the WRT54G v.2 (which is, at least for now, bricked).
With those two lights flashing, the router is reset to its base state. This is achieved by not mounting the /overlay/ FS, so all your changes are (only temporarily) gone, just the original OpenWRT flash image is available. This means wireless is off, ssh is off, telnet is open, and there's no root password. If you need to modify /overlay/ (usually the case), run mount_root, fix the problems, and reboot.