UV Packing to Fill Holes

I'm curious if anyone knows of a script that can take UVs currently in the UV 0-1 space, lock them, and then fill in the gaps with remaining UVs that aren't already placed in the O-1 space? Basically, I have sections of my mesh with a set texel density that I want to keep in the place they are in the 0-1 space and then fill in the remaining parts of the mesh automatically however it needs to. The script doesn't need to keep all the stuff at a uniform texel density, just pack it in as dense as it can without disrupting my current set up. Is this possible?