You are right, using the GPS tracklog to add the geotag afterwards would be less effort while achieving the same result.
A common problem with this approach is most cameras RTCs drift quite quickly, but sx260 can set it's time from GPS, so you could just sync it once a day or something and be quite close.
Another option could be to use a SBC to send the GPS data to the camera using PTP. I'd still need to work out the geotagging part. It's also more hardware to cart about.
Yes. A raspberry pi zero should work pretty well for this. You could also transfer the images to the SBC and tag them there, which would let you use standard software like exiftool or whatever instead of writing your own.
There would be applications where a functional UART is useful, for instance interfacing with a microcontroller.
No argument there. I'm not trying to discourage you from taking this approach, just suggesting alternatives if you aren't in a position to do the reverse engineering work.
The Canon UARTs are difficult to access, but I believe some Samsung cameras have the UART pinned out on a large connector.
May be true, but most of the work to set it up for CHDK is unlikely to be relevant to Samsung cams.
