As Rick said, this isn't yet implemented. In my use of stim_times_IM, we've resorted to getting the sub-brick labels:
3dAttribute -ssep ' ' -name BRICK_LABS dset+tlrc
And then divided it up using Python. Some relevant functions: string.split(), string.sort(), ",".join(string). Take the output string for running your 3dmaskave command.
Truthfully, the more straightforward way is use 3dROIstats to export to text and then use Excel to sort columns. Assuming your GLT names aren't too long to fit, you could do your entire study in one export and a few column sorts.
