You can use your Pixhawk to record camera trigger events indeed, if you use the hot shoe method (Enhanced camera trigger logging), you can get pretty accurate timestamps. The timestamp will be generated by Pixhawk, but Pixhawk syncs itself with GPS at startup, so it is pretty much the same.
It is possible, max. RTK rate is 8Hz, so you can use a GPIO input to detect camera shutter and then interpolate position depending on previous and next positions. There is also a possibility to generate the timestapm on TinyRTK directly by using the EXTINT pin.
Also possible, the timestamp can be generated with the EXTINT pin and then a little post-procesing will be needed to calculate the position due to the 8 HZ rate limit.