Given the discovery of extensive sulphates and essentially no carbonates, then SO2 might well be more likely than CO2!

It would behave in a similar fashion, of course, and you could have cyoclastic flows based on frothing SO2. Extensive sulphate alteration of bedrock appears to require more oxidised SO3 - the precursor to sulphuric acid.

Mars is certainly richer in S than Earth by about 4 times, according to nice work by Claude Allegre*. Driebus and Wancke** record elevated S and C contents in Mars meteorites compared to similar Earth rocks. They prophetically forecast in 1992 a sulphate-rich Martian surface.

"Recently, Wanke
et al. [11] emphasized the importance of SO2 on Mars. If we take
180 ppm H2O in Shergotty at face value and compare it with the
1330 ppm S and 620 ppm C in Shergotty, it is evident that sulphur
and carbon, respectively SO2 and CO2, are the most abundant
volatiles in the Shergotty magma. Under martian surface
conditions, SO2 will be transformed quickly to SO3, which in turn
will react with H2O to form sulphuric acid. Hence, it seems
likely that in addition surface water became acidified."

Jeff Kargel gave an excellent presentation at the 2004 LPSC, Mars as a salt-, acid-, and gas-hydrate world.

So, you're not totally out on a limb, there!

*C.J. Allegre, J.P. Poirier, E. Humbler, A.W. Hofmann, The chemical composition of the Earth, Earth Planet. Sci. Lett. 134 (1995) 515–526.

**in:- http://www.lpi.usra.edu/meetings/programs/msattcwwa.txt

[11] Wanke H. et al. (1992) LPSC XXIII, 1489-1490.