That's correct, if you are on a parking spot that is also used as a regular parking, Deicing there can only be done in the standard way: as part of the Departure procedure.
However, there's a way to make it work without completely disabling the parking spots (which you can't really do in GSX, it would require editing the scenery .BGL), and it's to use the "Distance Threshold" parameter in parking customization page, together with a custom position of the parking itself, so you can lead GSX to assume you are "outside" that parking spot even we you aren't.