Wouldnt any texture mapped to an object’s lightmap UVs provide the same pixel density info as a baked map? It sucks that, when I change the resolution on an asset (that had too much or too little lightmap resolution), it just turns beige and doesn’t tell me if what i did was enough until after I run an entire bake again. i remember UDK had an option to build only selected objects; that would help a lot if it came back in UE4.
This was intentionally changed a while back, sometime last year. The light build is required now to see the results in the Lightmap Density view mode.
This is an artifact of how the viewmode is implemented as it relies on built lighting data. There is the possibility that it could instead rely on the parameters that go into the light build, but this has been backlogged on the original ticket (UE-4451) that was entered with this.