nathanielbabiak 2020-11-01 03:41 (Edited)
This is the Pxl Library tool, for use with Pxl Library 2.0 here: https://lowresnx.inutilis.com/topic.php?id=1501.
It will save ROMs (or load them) just like the LowRes NX BG Designer. It will allow you to create or modify backgrounds (which include pixel-level graphic windows) and Pxl Library metadata.
There's also a whole bunch of additional explanation for how this whole thing works at the beginning of the source code.