Yes, the rental units should be able to lock the doors. There is supposedly a way to combine locks and rental boxes as well.
For guard robots, what about geese? They are traditional, after all.

I was thinking more in terms of indicating by layout and grouping which areas were public and which private, but that might be something we could discuss in the meeting on Saturday.
If anyone wants to have a go at experimenting with layouts on paper, the sim is a 256m x 256m square. (0, 0) is at the SW corner of the sim. Sea level is at 20m. The bulk of the sim land mass is currently at 22m, or thereabouts.
Rough sizes of structures:
Jim's cottage, Miller's cottage, Chapel cottage are each 10m x 20m, and about 12-14m high.
Lightkeeper's cottage is 20m by about 25/30m, about 20m high.
The carriage house (village inn) is 30m x 40m, 32m high.
The lighthouse is 14m x 32.5m, and the tower is 55m high.
The ship's hull is about 16m x 64m, with the pointy bit on the front sail making it about 82m from front to back. The main deck is about 15m above sea level. The top of the main mast is about 60m above sea level.
Since the ship is the biggest structure, it makes sense to start with that and develop the layout and terrain from there.
To help start to visualise areas, I've built a very rough skeleton ship with sticks and cones in the position that makes sense for sightlines from the rest of the sim. It helps to see on the ground the size the cove will need to be. It assumes the ship will be anchored along the southwest line, so at 45 degrees to the west and south boundaries. The pointy bit of the ship's front sail is tucked into the SW corner of the sim. The landing point for the ship would be to the south.
If we have a little harbour it could go along part of the south boundary, and the cliffs are along the west boundary towards the north to protect the ships from icy blasts. There will need to be adequate waterways (depth and width) all round the sim to let sailboats through -- we'd need to check this with Lightkeeper's sailboat, which uses very clever modelling to handle like a boat in RL.