I really like the idea of being able to lock objects as well as doors. I, personally, don't want to have to put my fridge in household inventory every time a guest pops in. Locks have the advantage of being optional so when you have a more long term guest or family member over, they can be unlocked. Or never used at all if that's your personal style.
I know for my families I make several meals at once to have on hand. Not all of them can afford to have those meals eaten by guests. It would be a financial hardship.
I think this would be a great mod if it is possible to do. I know there is a mod that allows Sims to claim some objects such as computers for personal use, similar to how beds work. But a mod that locked them to only household members would be great.
One work around that I have used is to add guests to a group. That way you can control activities better. I use this most often for kids coming home from school with my Sim kids. Usually I have them grab a snack together, do homework together, then do some other fun activity together.
The only problems I have found with locks is that as a rotational player I forget to unlock them before switching households. Then when I bring a Sim to visit, the host will often be locked inside a bedroom on the computer. Lol, then I have to temporarily switch over to unlock the door. But wouldn't it be nice to have an unlock all locks button like we have to turn lights off and on?