@Kent1Cooper wrote:
.... But I'm not sure how entity handles are affected by something being nested inside a Block definition. ....
Unfortunately, I find in a quick experiment that when I check the handle of a Line, then include that Line in a Block definition, and Insert that Block, getting the entity data from the nested Line with
(entget (car (nentsel)))
reveals that it now has a different handle. However, when I close the drawing and re-open it, the nested Line's handle still has that same changed value. And that's true in each occurrence of that Line nested in multiple Insertions of that Block.
So if, rather than save the handle of the Line before putting it into a Block definition, you can get its handle from within the Block, it should be possible to find that again later by stepping through the Block definition's pieces. HOWEVER, I find that with multiple coinciding objects [such as your Line along the edge of a Hatch pattern], you don't seem to get any choice about which object is seen when you pick on a location shared by more than one thing. So I'm not sure how to get the handle of the nested Line in a reliable way.
Kent Cooper, AIA