Is it possible to get the QNH value in Austria using a web service?
The current QNH at an airport is included in METARs so you can grab it form there. There are a few online services to retrieve the METAR automatically, such as this one (using Vienna airport as an example).

The latest METAR when I wrote this answer was:

LOWW 281050Z 14010KT 9999 FEW040 SCT065 BKN300 04/M03 Q1012 NOSIG


which shows a QNH of 1012.

Note that depending on the country the format may vary slightly, but in Austria (and European countries) you should always find a Qxxx value in the report.