Well, I had a bit of spare time, so...

ExifTool 5.93 (just released) has the ability to edit JFIF resolution information.

Thanks for your endorsement, it's nice to be appreciated.

Yes, it certainly has taken a long time to collect all of the information I have on various file formats (and even longer to reverse-engineer the formats for which I couldn't find specifications!). I now have quite a comprehensive suite of file specifications, plus sample images from well over 1000 different sources. I've often thought that I could save other people a lot of trouble by making these documents and samples publicly available, but unfortunately those darn copyright laws prevent me from doing this.