Since you're averaging numerous frames of video, you have very high dynamic range. Currently all you're doing is darkening each frame and adding it to the frame in memory. Why not set a max and min dynamic value as a percentage of the entire dynamic range of the composite?
For instance, say you're compositing ten video frames for each time lapse frame. And each incoming video frame has a dynamic range of 10. That means the dynamic range of the composite could be 0-99. Then all we would have to do is say "black is 10% of the dynamic range and white is 97% of the dynamic range"
This would get rid of a lot of the washy lights the come out in a lot of my footage.
James Thomas commented
I have a kind of similar suggestion: "More blending modes, like lighten for light trails/streaking lights."
The current blending is like "normal" or "mix". HDR may be possible and good too, but I think a "lighten only" blend mode would as fast (or nearly) to process than the current blending.