Here's the breakdown of what I'm dealing with. I've got DV and HDV YUV footage whose luma ranges from 16 to 255, which means it exceeds the spec by going over 235. I've been making use of the following syntax to convert this to RGB and preserve those whites:

ConvertToRGB(matrix="PC.601")

This creates video with luma values from 0 to 255, but the detail in my videos remains between 16 and 255. I need to learn the syntax for telling Avisynth to evenly scale that 16-255 to 0-255, so that I end up with an RGB video whose detail properly ranges from 0 to 255.

It's true that this would be a lossy process, and I may even have to convert it all back to YUV when I'm done. But whether I do it this way or instead scale the detail range down from 16-255 to 16-235, there doesn't seem to be any real difference. At least this way, I would be seeing the full range of detail from within PPro.