"...I've tried applying the SusDp control before the gliss..."
And why enter the command before? You want the dampening directly after the gliss - That's where you should add it. If you don't like the results, then try zeroing CC1 again - Either way calls for a new event to re-set things following the dampening you want. That's the way MIDI works. With simple switching controls, there has to be an On at some point following an Off - and vice versa.
The reason dampening after the gliss doesn't have an effect on the decay is that the dampening controller only affects notes as they are being played. In other words, dampening is a MIDI On (CC68, 127) event that needs to be sent before sending the notes that you want the dampening to affect.
Moreover, CC#1 has no effect on GPO Harp volume, which is esatblished and controlled by velocity.
AH yes, of course, my bad - cc1 doesn't control Harp volume. Lost my head.
It's been awhile since I've used a Harp, but I thought that what I did was use the pedal, cc64 for dampening, applied at the point needed - not cc68. If I had time I'd fire up Sonar and check this--but running too fast this morning.
I automate the mixer in Sonar, so swooping audio volumes down is always an option as I work.