.. but am still blind to the solution.
I don't blame you. Filtering by coordinates isn't seen all that often but it
is built into SSGET.
You need a relational test looking something like this:
(setq sset (ssget "X" '((-4 . "*,*,>=")(10 0.0 0.0 10.0))))
^this will filter all entities with a group 10 point that has Z greater than or equal to 10.0
The relational test for each coordinate is separated by commas in the -4 group and the point is simply given as it appears in the entity list, except each coordinate is a criterium.
If you need to filter more point groups, you need to provide more -4 groups. For example, to filter both group 10 and 11:
(setq sset (ssget "X" '((-4 . "*,*,>=")(10 0.0 0.0 10.0)(-4 . "*,*,>")(11 0.0 0.0 10.0))))