Japan Post represents only a portion of shipping providers in Japan, for our idea to work others will need the same privileged access to the system: Yamato, Sagawa etc.
This will also require to alter the package label on the last mile because requiring the courier to scan every package or letter before they could even see an apartment number will slow things down to a crawl.
The relabeling to show the actual address is something that carriers are already doing for C2C marketplaces and their anonymous shipping features already.