Hi, I really like this idea too!
I am, however, quite a noob, so I don't understand why we could only use prerendered images.
Can't the mini thumbnail behave like a frame of the same (paused) video opened again? So, when I hover on the main video time bar, the second one skips to that same time (maybe only keyframes) and shows an 1:5 mini-frame.
I don't know the memory-cpu load of a double file open with a fast seek, though.
Sorry if this is total bul****t, I'm just trying!