Some updates:

Qt Components
According to this Tweet, regular Qt Components should be part of the software stack. This should enable easy porting (or simultaneous release) of Harmattan applications.

Python & PySide support
I asked about Python support on the Jolla IRC channel. It seems that as long it works on Mer, it should be no issue. There are actually already quite a few buildable PySide RPM Mer packages on COBS.

Nemo
Might be a good starting point to prepare for Jolla due to similar architecture (Qt, Qt Components, RPM, Mer based). Nemo supports the N900, N9 and can be alse run in a virtual machine on desktop.