Marc,
Late response on my part, but if I recall correctly Matrox was a very strong OS/2 player back in the day. Their native drivers did well, as does the SNAP support, specifically the current readme shows the following:
. Matrox MGA Millennium, MGA Millennium II, MGA Mystique
. Matrox MGA Mystique 220, MGA-G100, MGA-G200, MGA-G400, MGA-G450
. Matrox MGA-G550, Parhelia, MGA-P750, MGA-P650
The Matrox MGA-G450 & MGA-G550 are also supported in the MultiHead mode, although not sure if these came in ISA configuration.
Finally, are you limitted to ISA only, or does that board have VESA connector?