Need help from electrical folks.

[f5fstop]

It appears the dome lamp harness runs up the passenger side A-Pillar molding. At the top, there is a six pin white connector. You should be safe to tap into the orange wire on terminal 1 for the inadvertent power supply voltage from the BCM.
You can then run the wire across the front of the headliner, over to where you want to install the new dome lamp; just tuck it up under the front.
I do not recommend lowering the headliner, it is not easy; especially with a sunroof. A lot of trim moldings have to be removed, and the headliner is formed, so it does not bend to easily, and if bent to far, it creases.
This will not only give you power to your new dome lamp, but will also be controlled by the BCM, so if you should leave it on, with the vehicle ignition off, the BCM will shut it off after 20 minutes, to prevent battery rundown.