Yeah, I appear to have flashed this exact DD-WRT mini load onto the WNR3500L initially, and then used that to flash TomatoUSB:

dd-wrt.v24-14896_NEWD-2_K2.6_mini-WNR3500L.chk

Oh, wait.. If I had to guess, then the DD-WRT mini load above was actually then followed by a Tomato mini load, before finally flashing the main TomatoUSB:

tomato-K26-1.28.9054MIPSR2-beta-Mini.trx

Not sure why it was so complex, but I was religiously following directions from somewhere -- very reminiscent of DD-WRT installs, and very un-Tomato like. But it only needed doing once that way, and subsequent Tomato upgrades are simple web-GUI affairs.

Cheers