...would presumably be done before anything of the magnitude of a massive SO2 release went forward.  

As it happens, we already know the consequences (smog, acid rain, global temperature drop if scale is large enough) of a massive SO2 release, as you have pointed out.  So the modeling should not be too fraught with uncertainties.  

Carbon sequestration, which is on everyone's lips these days, is a most likely a problem with more uncertainties--how carbon dioxide will affect subterranean rocks and soils; question of leakage and release after a few hundred years, etc.