Just a kind of after thought:
If I were to listen to this kind of progression I would probably play in D major and treat the C as a bVII, mostly because I'm used to that sound in the type of music I play, and I'd treat the Bb as a bVI and imply the maj7 in my playing.
Sure I wouldn't play the C# in D and I'd play C and I'd be playing D Myxolydian technically, and I wouldn't play F# in the Bb so there I would be technically in D Phrygian.
My point is that I would probably focus on the D note throughout my playing and resolve to it.
What you're saying about starting in G major and switching to G minor at the Bb sounds cool, but, at least in the type of music I hear and play, I never hear a progression like that. As in V - IV - bIII
The I - bVII - bVI is way more common for me.
um thats all
[list=]I wanna be that guy that you wish you were ! ( i wish I were that guy)
You gotta set your sights high to get high!
Everyone is a teacher when you are looking to learn.
( wise stuff man! )