Is there a simple way of signalling that a new preset has been selected in the VCS?

No 'simple' way yet.  For now you could try one of two approaches:
1. Create a "dummy" EventValue called !PresetNbr and, each time you save a preset, set the value of !PresetNbr to the number of that preset.  Then you could always check whether !PresetNbr hasChangedInLast: 0.2 s
2. Put your Sound into an InterpolatePresets and always change the preset using the !Interpolate fader.  Then you can check against !Interpolate hasChangedInLast: 0.2 s