how about borrowing a concept know as master channel, which is implemented in a lot of multi-track audio editing systems ?
This master-channel, which basically contains the final rendering output, doesn't need to appear on the time line. But from as a concept, this output track should have it's own effect stack. Maybe the rendering dialog is a good place to link in this post-processing effect stack. It would allow to apply any sort of post-processing in a very customizable and transparent way : (de)-interlacing, noise reduction, scaling, cropping, color correction, audio gain, audio equalizer or compressior, ....
Of course, some of the options like deinterlace or scaling are already present in the rendering dialog. But the users has no control on the order effects are applied.
Putting this master track a the bottom of the time line and making it keyframeable would be the ideal solution. Why keyframes ?
Think of a common scenario: You've just finished a complexe edit containing of a large bunch of tracks and clips and notice that color balance or audio volume slightly changes from the start to end. Of course, you could go back and edit every single source clip locally. But a keyframeable volume gain on the "master channel" could do the trick in a single steps.
Of course, a master channel doesn't add core functionality to kdenlive. But IMHO it's pretty obvious that a master channel adds a efficient degree of freedom in editing. And I guess that's the reason, why you find this concept in a lot of prof. multi-track editing tools.
What do you think ?