Its simple: Leave the plots how they are, and from the same area you access normal housing, add an option to enter an instanced housing area specifically for yourself. In place of the Ward selection, have it list everyone on your friends list, party, or simply a search bar for someone's name, so that you can enter it while they aren't there.

Just reuse the same assets from the original housing areas so no new assets have to be crafted for it. Just make it at a distance far off that overlooks the normal housing wards or something.

There, everyone has a house, no new assets need to be created, and this whole availability thing becomes something of the past.