There have been prefessional productions of the show with mixed-race casts. Susan Moniz played Dorothy at the Marriott Lincolnshire Theatre near Chicago. The producton featured actors of all ethnicities. There were a few African-American actors who tried to mount a protest. Notably, these were actors who had never been cast at the Marriott (and for that matter, had no Equity credits in the Chicago area.)
Unfortunately, Tamyra Gray turned down the role of Dorothy...as did the R&B singer Brandy.
I'm all for the girl who is currently the understudy in Little Shop...Ta'rhea Campbell (i'm sure i spelled that wrong). Her voice is amazing!! She was once the understudy/standby for AIDA (on tour). Unknown and VERY talented!!