Hi,
I broke the config. Ok. Was trying to have guest vlans in this box and Kamikaze was doing fine but my wife's phone was rejecting to associate with the hidden SSID.
So I decided to upgrade to see if newer code had better WL support.
Tried two variants of Tomato, and both refused to program a trunk in the uplink, which I know works because I was using it in this same system.
And I read that 10.3.1 (BackFire) was ok in this hardware, and I flashed it.
Luci run ok, but somehow the code also was refusing to accept an uplink trunk consisting of two vlans.
(internal and guest, I'm using the AP just for WL, real border services are in a dedicated server).

So I moved to command line programming of the internal switch using nvram.
To be consistent with external switches, I renamed vlan 0 to vlan 1... and forgot to change the internal interface, so now the internal if is disconnected and the external ports link to wan interface (vlan 1).

Ok, let's failsafe and start over ? I can't.
I see the udp packet, but instead of "press now to enter failsafe" the contents is "dead".
I can not bring it into failsafe mode, it won't no matter the tens of times I've tried different timings.

I thought the console port was going to help, so I soldered a 4 pin connector and connected a serial port (using a level converter) and nothing. A scope confirms that it seems to be no activity in the serial port in all the boot process.

I know it is booting, power blinks, DMZ comes on, it even asks for DHCP (WAN) on all ports.
I don't have an LPT port in current machines, that I guess is needed for jtag, but it seems that is the only way out now.

Help ?

Strange that you don't see anything on the serial port.

It might have a CFE mode.  This is where you hold down the reset button while booting and the bootloader stops the boot and serves a web page that lets you upload and flash firmware.

Beats me, I don't really understand the whole loading sequence, and I had run into failsafe before.
In fact, it seems I have to pay more attention before trying to have it work the way I want
I've decided to apply a "last resort" flash address tinkering and it worked.
Due thanks to Voidmain's notes.

Now back to get my uplink trunking as I want it. If anyone has some pointers to a guest ssid setup on a WRT54Gv2, I'm all ears (AP only, WPA2, multi SSID)

Should be about the same as any other hardware.  You need to be aware of the port numbers of course.

Looks like you could:
first make a lan wifi AP and connect it to lan.  Then you can use it to log into the router even if you mess up the ethernet ports.
change the wan port (5) in the switch from untagged in 1 to tagged in both 0 and 1 -- this will be your uplink cable
delete the wan interface
make a new interface (bridged) called guests and connect to eth0.1
make guest wifi AP and connect to guests

I didn't think that VLAN0 works over cable you may need to re-number it to something else.  In that case be sure to change it in lan physical settings also.

Thank you for being here
Your plan is like I was trying to do, but I goofed about the processor vlan access change.

What bothers me (actually what I don't fully understand) is that the switch insist in tagging when I'm not asking to.
E.g. I now have:

# cat /etc/config/network
#### VLAN configuration
config switch eth0
        option enable   1
        option enable_vlan 1
        option reset 1

config switch_vlan eth0_0
        option device   "eth0"
        option vlan     1
        option ports    "1 2 3 4 5"

config switch_vlan eth0_1
        option device   "eth0"
        option vlan     2
        option ports    "0 5"

config switch_vlan eth0_4
        option device   "eth0"
        option vlan     4
        option ports    "4t 5"

#### Loopback configuration
config interface loopback
        option ifname   "lo"
        option proto    static
        option ipaddr   127.0.0.1
        option netmask  255.0.0.0

#### WAN configuration
config interface        safe
        option ifname   "eth0.0"
        option proto    static
        option ipaddr   192.168.1.1
        option netmask  255.255.255.0

#### LAN configuration
config interface lan
        option type     bridge
        option ifname   "eth0.1"
        option proto    static
        option ipaddr   172.30.0.213
        option netmask  255.255.255.0
        option gateway  172.30.0.1

#### WAN configuration
config interface        wan
        option ifname   "eth0.2"
        option proto    dhcp

#### GUEST configuration
config interface        guest
        option ifname   "eth0.4"
        option proto    static
        option ipaddr   172.30.3.2
        option netmask  255.255.255.0

# cat /proc/switch/eth0/vlan/[0124]/ports

1       2       3       4t*     5t*
0       5t
4t      5t

Notice the "safe" if just in case, although I did not need it.
But the vlan 1 is being tagged even though I did not ask for that in the config file.
I'm looking forward to convert port 4 in a trunk, with vlan 1 native.
Will change the switch to tag everything and hope it will work, but it is not obvious...

Sigh,
the upstream switch (a MicroTik switch) refuses to "digest" the vlan tag, so as the frames get tagged, then the hosts do not understand them

And I don't get why from my "1 2 3 4 5" line, the switch gets "1 2 3 4t* 5t*".
If I write "1 2 3 4 5t*" it works, so I guess I will stick that into an rc script. Ugly.

It seems it won't work.
Whenever I config a second SSID, the wireless does not work.
So I gave up on the WRT54G, and tried another AP (ADB A4001N), also based on Broadcom radio.
And b43 code which does not support multi SSID
I could not make it work with brcmsmac either. And I could not find any compatible broadcom driver.

Giving up un multi ssid for the time being.