It has been a long while
And today I had need of this, and decided to try again. I think I got it working!
It's barebones and not tested extensively (it's waay past bedtime!), would love to hear if there is something I missed!
I got the solution from: http://www.flipcode.com/documents/matrfaq.html#Q40
Q40. How do I use matrices to convert one coordinate system to another?
extractTransformMatrix_v001.hipnc