I've probably overthought this and missed the obvious--I have multiple different groups of points (a changing number, this has to be scalable), and I want to create polylines between the border points--essentially connect adjacent pieces, but only pieces that are not in the current group, instead of only pieces that are.
(The end result is creating prims to use as glue constraints in a grain setup connecting clusters of points. Because there are gaps between the chunks (and those gaps are where I want to form my polylines) using the connect adjacent pieces doesn't work because all adjacent points are found before the necessary distance has been covered. It's for a softbody tearing sim using grains for point deformation to compensate for vellum welds not accommodating solid objects--project file in this post:
)