I myself had a issue similar to this today. When i tried accessing internal methods using QTP it crashed the application and at times I used to get a Access denied error stating the calling thread and current thread is different. I believe QTP has some kind of problem around this thread area in WPF
Good luck with QTP and WPF. HP's failure to provide robust and comprehensive support for WPF in QTP is the primary reason I am transitioning all future automation development to TestComplete 7.
QTP fails to provide adequate support for the "standard" WPF objects they claim to support, and completely fails at supporting customized or third-party WPF objects. With the current version of TestComplete 7, we not only get good support for WPF, including some non-standard and third-party controls, we also get support for .Net 4.0 controls (currently in beta) and Windows 7.