this is a good question, maybe should lead to a feature request.
without an actual option for controlling the .tvg quality, what you might be able to do as a workaround is set up simple pegged rig with the quality-reduced bitmap "clone"of your .tvg background on a different layer, and then swap out the visibility of this proxy bitmap for the .tvg when you are ready to render. that way you can manipulate your background, even with parallax etc., via the peg( s ), and the manipulation keys and stuff will apply to both the proxy and your final HD background. that’d be my solution, again without knowing a way to set/script the .tvg quality.
there may be a clever way to do it in the network view, but i’d imagine anything there would be an additive process and may simply cause the machine to do even more processing toward the high-rez .tvg.