I just installed Tomato Firmware 1.28.0000 MIPSR2-121 K26AC USB AIO-64K
And I found where I can get tcpdump at this thread:
martin-denizet dot com /run-tcpdump-tomato-firmware-without-thumbdrive/
Says that this was tested on v116 by Shibby.
Sadly, when I start up tcpdump, it does seem to work, but after 20 packets or so it crashes the router and it restarts.
Is there a newer tcpdump that works with v121, perhaps?