Set a factioned entity to personal, with you as owner. When you leave the faction, that entity will still be set to "personal" and you as owner.
However, this causes 2 issues:
- You as the original owner can't edit it anymore, it's not your faction.
- The faction can't edit it either, they're not the owner
When you leave a faction, your personal things should go with you (or founders could steal stuff from you that way).
A possible fix would be that the "personal" permission overrides the faction signature. If something belongs to another faction yet has "Personal" and you as owner, you can override the faction and edit/remove/use blocks on that ship.