I have another option, wondering if anyone knows whether this is possible.

If I book a return flight in WT+ to MIA, then on the return I choose the option with a connection at BOS, if the BA leg of that journey has reward availability in CW, will I be able to upgrade with miles?

Or is this not allowed on a multi-part sector? I know I should ask BA, and I will ultimately, but they too-often hit me with an uninformed answer...