I have an idea how to simplify geosync option a bit. I record a gpx track with a mobile phone. If I show a QR code with current time on the phone screen and take a photo of the screen with a DSLR, I get both phone and camera time in one image file. To check the idea I made a page with javascript showing such QR code and patched another piece of software to support it. It now works for me fine but I guess exiftool could also benefit from this.
Interesting suggestion, but it seems like your technique involves analysis of the QR code in the image, which is something that ExifTool won't do.
- Phil