OpenStreetCam images in JOSM without upload

I am not aware of a plug-in for JOSM that can deal with mp4 files. You can extract the images from a "normal" MP4 file using ffmpeg but those images will not be geo-referenced. There are some tools that allow you to add gps exif data to photos but you'll need to get that somehow from the MP4 file. But, again, I don't think there is a standard for how GPX data can be encoded in a MP4 file. And it is not clear to me that OpenStreetCam's MP4 file adheres to standards.

For what its worth, I was dealing with a similar issue when setting up my "dash cam mapping" methodology. Fortunately for me I found some code on the Internet that was able to extract the proprietary format GPX information from my dash cam's video. I used that along with ffmpeg and exiftool to create my dashcam2josm script.

In my case, I happen to use the scripts your link points to up upload the still georeferenced images from my dashcam2josm script to OpenStreetCam. I do that because I figure there are things that I am not worried about mapping (fire hydrants, bus stops, etc.) that others might want to have imagery for.

Getting back to your issue, it looks like the generate_exif script in the https://github.com/openstreetcam/upload-scripts repository may be what you need. But it has been so long since I gave up trying to use the OpenStreetCam app that I am not sure what files it generates and don't really know what the script help message means about "Folder PATH with OSC metadata file and images". Can you extract the images from the file using ffmpeg? If so then maybe the generate_exif script would then add the GPS data to them. . . Just a guess though.

upload-scripts$ python osc_tools.py generate_exif -h
usage: python osc_tools.py generate_exif [-h] -p PATH [-l {d,i,w}]

optional arguments:
  -h, --help            show this help message and exit
  -p PATH, --path PATH  Folder PATH with OSC metadata file and images
  -l {d,i,w}, --log_level {d,i,w}
                        Level of logging to console:
                          d level (debug) will log every event to the console
                          i level (info) will log every event more severe than debug level to the console
                          w level (warning) will log every event more severe than info level to the console