Really? I always thought that "ricer" was a word play on "racer", a funny term used to describe idiots in their pimped out cars who think they are racers.
(This is more my personal opinion, and less sourced)
I think the "rice" word (aside from the racial connotations) came from the stereotypical first modification being an absurdly large exhaust to try and lower the frequency of the typically smaller, higher-rev'ing / turbo'd Asian engines.
A noticeably large cylinder shoehorned on to the back of your car? Rice cooker.
