Can you Mask an Image Service in a Flex App

I have a basemap service that covers two towns. I have a few flex apps. One of the apps I want to be geographically restricted to only 1 of those towns. I have boundary layers to delineate the towns. Without creating a new basemap service, is it possible to mask out the part of the basemap that I don't want seen?

I know there are sublayer definitionqueries to mask out vector layers, so I'd to do something similar with an image layer, but based on a spatial query. I'm running 10.1.1, Flex 3.2.