Ok, I just took the utility I wrote that turns off partially dead pixels, and modified it that you can merge one fseq file with another to give an overlay.
It will overlay anything with a > 0 value.
If anybody is interested, binary and source on:
https://www.dropbox.com/sh/y40moacu4rlpd4d/AAB7EPymqCD5y6Xep1h_LjkOa?dl=0
Just drop fseqfix.exe and fseqfix.json into the same folder as the fseq's, modify fseqfix.json as needed and run fseqfix.exe before uploading to FPP.
E.g.
[{
"files": [
"All I want for Christmas.fseq",
"Kiss Me Babe.fseq"
],
"merge": [
"TuneTo.fseq"
],
"rules": {
}
}]
Make backups! This modifies your fseq files in place.