Since I found out, I can share as well.
Use the RefValue or Text property in the definiton file, let QV generate a property page for you (QV 11 that is), copy, rename and modify this qvpp file and point towards this modified qvpp file in your definition file.
This is though (at least in 11 SR1) buggy so I got it working using this syntax:
<PropertiesPage Version="11" File="DynProp.qvpp" />
first in the defintions file. Do not skip Version="11" even though documentation says its not mandatory.
Read by using Layout.Text0.text but write to Chart.Text.0.Content...
I would like to see this in the documentation QlikTech
Pls also note that SetProperty does not work really good in 11 SR1, better in 11 SR2.