Nice question BD. First, the spalted maple is a little thicker than the flame veneers, so that helps.
Unless there are scratch marks or glue spots to contend with I do not sand the veneer prior to applying the timbermate.
My rules for sanding veneers are to use 320 grit (or 240 v e r y sparingly). its actually not easy to sand through the veneer with 320 or 360 grit.