I managed to get my altitude to display by using a pythonprogram(https://github.com/dunz0r/KCC/blob/master/Display/printToSerial.py) which reads data published via an http-socket from the awesome KSP-plugin by Richard Bunt called Telemachus(https://github.com/richardbunt/Telemachus).

The program's fairly ugly at the moment, but it will become more polished as I go along.

Below is a video showing the altitude on the display