When you import the OBJ file then either save the scene or save the prop to the library, Poser saves the geometry data within the PZ3 or PP2 file. Reloading the scene or library prop will not reference the original geometry, Poser simply reads the data from within the file. Which is why currently you need to delete the prop to effect any changes.

Suggest geometry stripping:-

• Import the OBJ files in the order that you require.
• Save the PZ3 or PP2 file.
• Use PZ3editor, (I believe you have a copy or other similar utilites exist), to extract the geometry from the file.
• Save and continue in Poser with the stripped file.

You'll still need to save and restart Poser to recognize changes to the geometry but the file will now be referencing the remote geometry rather than any internal data.