For those of you that can handle MFX plugins (Cakewalk and recent Cubase offerings)...
As of v4.0 my MFX SustainFix plugin allows "Fake Hold Pedal Functionality" to operate on synths that don't normal respond to your hold pedal. This means that you can keep your default GPO Sustain Pedal Mode to affect legato switching, and use this MFX plugin as an insert to allow Piano sustain playing on particular tracks (without having to have two instances of the player on your system).
Give it a whirl if it sounds like you can use it. Download Here.