I see the analysis of the image and the TES measurement of this area at the time the image was taken on your page:

Flow Conditions and Possible Mechanisms
http://irian.geology.latrobe.edu.au/~nhoffman/Mars/Polar3.html

But I think it would be difficult to determine the time when the flows actually occurred unless you find before and after images of the same area, which you acknowledge is now lacking.
However, if as you say such flows occur annually in this location this should be something MGS can specifically look for. Note also that the upcoming report in Science by Costard et.al. also suggest the flows are due to melting ice, though in their theory melting water ice. Then finding before and after images and noting the time and temperature at which they occur should allow us to distinguish which volatile is responsible for their formation.

Bob Clark