Hi Karsten,
after AHRS data support – i.e. the attitude information for the artificial horizon – was introduced in VFRnav 4.15, I’ve now been able to look into your second suggestion as well.
My latest finding: Stratux already provides additional altitude information in the NMEA sentence $PGRMZ. If a baro sensor is fitted, this value corresponds to the baro altitude referenced to the standard atmosphere. That would be the perfect basis for an external pressure measurement for the altitude display in VFRnav.
The problem is: if there’s no baro fitted in the Stratux, it sends the GPS altitude in the same sentence.
I therefore still need to examine more closely how to reliably distinguish the source of the PGRMZ altitude. Maybe someone has an idea?
Why all this?
With the current pressure, which could be derived from the PGRMZ sentence, VFRnav could display the baro altitude according to the entered QNH – which, as we know, is much more accurate than GPS altitude. In addition, a variometer could also be implemented this way.
That’s it on this topic for now.
Enjoy your holiday @KBfly 
Hermann