Solved

When I got the new version far enough along, I did some diffs with the original failing version. I noticed an error in the part.fzp file. I knew I did not want real schematic and pcb views, but somehow I had duplicated the icon svg references for those layers, instead of the breadboard svg. Swapping that got the failing version to work.

2-by-x-header2breadboard.fzpz (2.9 KB)

Now I can expand it to the full headers / carrier I want.

In the process of testing though, I confirmed that this only works when the part family is set to Breadboard. Anything else wants to put a representation for the part on the schematic and pcb views. Instead of just the (rats nest) wires.