Well, after a few miscues, it took.

If you do a TFTP -i -v 192.168.11.1 PUT dd-wrt.v23_generic.bin, then you
will see some boilerplate.

Then when it starts to transfer, it will say:
Packet will be sent. len=60, opcode=2
Then it should say the identification of the block that is being sent.

If you see this, it probably means that it's not taking, and will fail.
Packet will be sent. len=60, opcode=2
Packet will be sent. len=60, opcode=2
Packet will be sent. len=60, opcode=2
<repeats another 8 times>

Now I have flashed it, ipconfig /all reports CONFIG2 as the server, I
think, the port blinks, but no joy at 192.168.11.1

Perhaps it's due to not logging in under W98SE (kiddie OS)? I'll swap it
out with the one on the cable modem.