How can I get a beam to link notes running upwards in the lower Staff to a continuation of the run in the upper Staff
Basically, you can't. Go to the wishlist and add your voice to the requests.
There are some weird tricks that sometimes work, using non-visible notes to force beams to meet in the middle, but they don't often give particularly good results. I wouldn't hold my breath for a solution, however, as in NWC there is a only a slight connection between the staves, just enough to keep bar lines and notes lined up vertically. To date I can't think of any feature which allows a graphic to span two or more staves. (Grand staff braces, I suppose.)
That is whatI thought. As you say, add to wish list!