I can also suggest a whole NEW approach that may cost you absolutely
nothing.

........... invite the biggest people you know over to sunbathe in
their bikinis. Speedos?

........ have parties where everyone must bring binocs and spend at
least 15 minutes using them on the house up the slope.

......... do something, equally obnoxious in the back yard that is
still within the rules of the covenant that will force the owners of
the church oops, McMansion to put up the barrier, which is infinitely
easier on their upslope than it is on yours. In fact, before doing
ANYTHING go have a talk with them about privacy and it is 1. likely
they are planning some privacy plantings already and 2. if not,
suggest a cost sharing.

Ingrid