When I sent the email, I recalled that several chess960 simuls had already been played at Mainz. My overview of past years, Chess960 @ Chess Classic Mainz, points to two pages: Playing Chess960 Simuls on 20 boards: Leko & Svidler (2003) and Chess Classic Mainz CCM6 (2006). On the page for 2006 there was no info about Aronian's 20 board simul, but it did get a mention on the corresponding report by Chessbase.com: Chess Classic: Chess960 results, Anand and Aronian simuls.
After a little more digging, I discovered that 59 of the 60 simul games have been preserved. The missing game appears to be Aronian's win against celebrity 'German soap star Vaile', mentioned in the Chessbase report. I also discovered that, in More on the Concept of Distance, I had already commented on an Aronian simul game with celebrity supermodel Carmen Kass.
From those game scores, I can answer two of my original questions. Many of the opponents for both Leko in 2003 and Aronian in 2006 are listed with their ratings. There were a fair number of 2000+ players in both simuls and of the other players with ratings, all were club players or better (>1400). Of course I can't tell if they were experienced chess960 players, but in 2003 I would guess that most were inexperienced. There were also a few chess celebrities among the GMs' opponents. Leko played against Albert Vasse of DGT, while Svidler played Carsten Hensel, Kramnik's former manager.
The PGN records show that the GMs had to contend with a different start position in every game. This was confirmed by the 2003 page:
It was not an easy task for the two top-players, because they could not make use of their enormous opening knowledge. However, their general understanding of the game was enough to win the simul comfortably against 20 ambitious club players.
Now if I can determine how the start positions will be selected, I'll know (almost) everything there is to know about chess960 simuls at Mainz. Perhaps they will use the DGT960 clock, 'the only chess clock worldwide that automatically generates the starting positions of chess960'.