According to the results sheets I have, there were at least 4 RT21s in Australia for the start of Formula Holden in 1989. One for John Briggs, one for Tony Blanche, one for Simon Kane, and the other was the Dave Mawer car.
The Simon Kane car was also driven by Neil Crompton, in a one-off drive at the 1991 AGP meeting, where he finished second.
The Tony Blanche car became one half of the Thalgo-backed team in 1990-91, and was driven by Blanche, Mark Larkham and Mark McLaughlin.
The Dave Mawer car was driven by various drivers, including Elwyn Bickley (at it's first race), John Smith (who damaged the car very badly in an practice accident at the Phillip Island round in 1990), sports sedan racer Keith Carling, and Rohan Onslow.
I can't find any record of anyone other than John Briggs driving the John Briggs car
Oddly enough, I can't find any record of all four cars appearing in the same race until the 1991 AGP meeting (however, I don't have results from the 1989 AGP meeting).
Also, as a sidenote, I was watching my tape of the first FH race today, and John Smith (who was guest commentating) mentioned that Rohan Onslow's RT20 was an ex-Pavesi car.