I think most people are noobs when it comes to this, no ones documented anything about it.
Well in NSE if you click ctrl-m it will go into advanced mode and show additional information.
There will be a label that says Pal Register ## ## ## ##.
Then you will want to open up NSE's built in hex editor and click sprite offset and find the pal register that was shown.
You probably will not have to scroll at all.
You will want to edit that first number to a free register. use the list I posted on Q&A for a small little pointer in the right direction on free registers.