Hi Joseph, the solution I found was this:
The ‘GlideSpeedBoost’ parameter can be set to -100 to 100, and when the event is triggered this param will increment both GlideBoostInc & GlideBoostDec - however their curves are set up such that values of -100 to 0 will only affect GlideBoostDec, and 0 - 100 only GlideBoostInc. Both these params have velocity set to bring them back to 0 over time. This has the behaviour I intended: repeated triggers of this event will increment/decrement by further amounts.
Unfortunately this has now raised a potential bug as mentioned in this thread: Automating the AHDSR envelope of a parameter with another parameter causes Unity to crash