Could you just attach a smartphone to a door, and have open one of those Supra lockboxes over Bluetooth? I guess the issue there is battery life.

I make prototypes of stuff like this as a hobby. You could use a fairly inexpensive SoC that has Wifi/Bluetooth built in ($6), attach a module for 4G connection (~$50), and give it a huge battery so that it can lasts a few weeks. It would be asleep most of the time, and wake up every 2 minutes or so to check for updates. Meaning you'd have to stand there at the door for a couple minutes before it'd give you access.

The cell phone + supra lockbox approach could work. One would need to be a realtor though. Supra only sells through realtor associations.

Thanks for the info regarding the components.

I wonder if there'd actually be a market for this kind of thing.. i.e. One person unlocking access to physical keys in a lockbox at a remote location so that another person can gain access to something. It would be used in a temporary installation, not like ring.com installed at a front door of a house or remote access tech built into cars these days.

Another case of technology in search of a problem... :-(

We actually built something like this but for bikes. http://lockandgo.bike/index_en.html, it has data connection for remote lock/unlock and even gps to get the position. There was no market. We could pivot to this easily...