That would be easy enough to do. Not the early access to BG, per se, but allow Rasaad's quest to be done early. You just need to move the events in BG to Nashkel and perhaps, Beregost. Not very complicated, and maybe a nice little mod to get your feet wet. We need more story modders. Come! Join us on the dark side!
Since you show no signs of doing this, I'll add this to my Themed Tweaks mod. It wouldn't leave my brain. IT. MUST. BE. DONE. But if you are at all interested in writing it, I'll leave it be.