I was sceptical when I first read this so I tested it out because the .rpl format only saves whole frames.
I moved exactly the same joints but at different times during one particular frame (490)
Nr 3 is without, let's call it "frame manipulation" and Nr 4 is edited three quarters into the frame (490.75)
If you open them both in notepad you'll notice the numbers are exactly the same.
The conclusion becomes the following:
While you can use this to edit the replay at a much more specific time, after you've saved the replay it won't make a difference because the .rpl format is limited to whole frames. So the engine itself can handle this new cool thing but it won't make a difference because the output file doesn't support it.
I assume this will conflict in one way or another, but the glitching is probably so small it can't be seen with the human eye and because we can't check for glitches in notepad either (because it's a .rpl file) we may never know.
Cool find though, I like discussing stuff like this.