I can confirm that this is currently not supported by IPAM. We didn't get the strong uses cases for documenting non used IP addresses. The one thing you may do is to set IPAM not overwrite custom properties when IP address goes from used/reserved/transient to Available status, by ticking "Update but not erase manually entered data" checkbox in subnet settings:
Let me know if there is a use case where you need to update custom properties for Available addresses more frequently. We currently track this as a feature request but don't set high priority for this.
thanks,
michal