Change export location dynamically in an export service (SDK)

I have a plugin that provides an export service. I would like to have presets that get mapped to different subdirectories ("Galleries"). I set the destinationType to specificFolder, but then the preset that I save from the UI has a hard-coded path in it. I would like to be able to change the 

LR_export_destinationPathPrefix property to a different path that I compute based on the name of the gallery chosen. Is it possible to do this from the startDialog(propertyTable) function that I define?

BTW, the LR_* properties are all in a subtable of the table passed as an argument to startDialog, while my custom keys are at the top-level.