[desperate]

OK, small update:

I've managed to make the Indy attempt bootp, so I can verify that I've managed to enter the Command Monitor. No need to set any IP address with "setenv netaddr", just following these steps:
1. press Esc immediately after hearing the boot tune.
2. press "5" and Enter.
3. run "boot -f bootp()/blah".
tcpdump output on laptop shows that the Indy first sends an ARP request and then four bootp requests to 255.255.255.255:67.

So now I know I'm in the Command Monitor, but running "unsetenv monitor" doesn't make the monitor wake up, nor does any of all the combinations of "init" and "exit" and "off" and rebooting and reentering the Command Monitor that I have tried.

Of course there's always the chance that the cable doesn't work, but given the error message on the Dell monitor I think it's working, it's just that my Indy stubbornly refuses to send a signal that the monitor can handle.

Any ideas?