I think I figured it out. (Amazing what you can do when it rains all day). I can drain the lav directly into the main stack, BUT- I will have to vent it separately, since there is a toilet above it. I really want to add a laundry tub, too, so I'm going to come out of the main stack on a 45 degree to the right to clear the radon vent, then another 45 into the future wall on the right. From there, I'll connect the lav (in the bathroom) to the laundry tub (outside the bathroom) with a back-to-back san T. Going up will be a vent, which will cut across the ceiling to the basement bathroom vent. Going down, I'll use a long sweep ell. Although the drain will be outside the wall where it cuts across the corner, it will be inside the vanity for the lav. Clear as mud? See any problems with that plan?