Hi Karsten,
after support for AHRS data – i.e. the attitude information for the artificial horizon – made its way into VFRnav 4.15, I was then able to look at your second suggestion as well.
My latest finding: with the NMEA sentence $PGRMZ, Stratux already provides additional altitude information. If a baro sensor is installed, this value corresponds to the barometric altitude based on standard atmosphere. That would be exactly the right basis for external pressure measurement for VFRnav’s altitude display.
The problem is this: if the Stratux has no baro installed, Stratux sends GPS altitude in the same sentence.
I therefore still need to examine more closely how to reliably distinguish the source of the PGRMZ altitude. Perhaps someone has an idea?
Why all this?
With the current pressure, which could be derived from the PGRMZ sentence, VFRnav could display barometric altitude according to the entered QNH – which, as is well known, is much more accurate than GPS altitude. It would also make it possible to implement a variometer.
That’s it for now on this topic.
Enjoy your holiday, @KBfly 
Hermann