Rather than measure the pins of the SD card, measure the current draw of the camera as a whole to determine when writing data is complete. There should be a significant, easy to measure, drop in current when done. You could use that as the signal to gracefully shut the camera, e.g., retracting lens and closing external shutter.

You might consider adding a small solar panel to top off the batteries, at some expense in complexity. Even a few mA should be sufficient, at that low drain.

BTW, in extreme conditions, e.g. dark during storm or solar eclipse, the exposure might be longer yet, and noise-reduction could then double the write time, so simply relying on a fixed time, as you state, might not be sufficient.

Also, BTW, regrettably, "moving as slowly as a glacier," is no longer accurate. Over repeated visits to sites such as Mt. Rainier (Tahoma) in the U.S., I've observed a profound retreat of some easily observed glaciers.

Rather than measure the pins of the SD card, you might try to detect the blinking SD symbol in the viewfinder.

You also might consider adding a small solar panel to top off the batteries, at some expense in complexity. Even a few mA should be sufficient, at that low drain, and allow for a longer window for exposures to complete, even if you cannot detect end of write to card.