Frowned upon! Its downright bloody stupid! A pressure instrument is set to a known pressure or elevation if you have a reference.
no QNH can be obtained?
How long do you fly for? You have a met forecast, you took off from a known elevation so you have must a reasonable idea what the QNH is or should be. The Altitude on a GPS is the least accurate part of it!
I was once involved in Research flying where we operated to 50 ft over the sea, we used a totally non standard procedure of setting the pressure altimeter to the Radar Altitude as that was invariably the most accurate datum but even then wave height may have produced spurious readings.