Because IGC loggers are vented to the cockpit and respond more rapidly
to small pressure changes than a mechanical altimeter, you might be able
to test the pressure difference with your logger. While flying at a
steady speed, open/close the vent, while noting the change in altitude.
Later, you can convert the altitude change to pressure changes.

Well that probably won't help me as much since i want to compare
between 2 different flights. The configuration change is not easily
done while flying so i wanted to compare two different flights.
But I might be able to open and close the front vent and see if there
is a pressure change on my volkslogger at a certain airspeed and
altitude. Then look for the delta on the next flight.

Chris