You would have to determine the dimensions on your server (with ffmpeg or mediainfo or something similar, or by hand) and pass those dimensions to your brightscript program with XML or JSON metadata, and then see if you can adjust SetDestinationRect to compensate.
Best would be to encode your content in the desired aspect ratio to eliminate that, or to use source content that is of the desired aspect ratio.
Also if you know all this beforehand and the data is essentially static you can overlay attractive borders or a rectangle Border the same color as the screen below. If fullscreen you can still overlay Something more appealing. If using image canvas set your border layer above The video. If roscreen set the sprite z order above the video
My Channels: 2D API Framework Presentation: https://owner.roku.com/add/2M9LCVC Updated: 11-11-2015 - Completed Keyboard interface The Joel Channel ( Final Beta )