Although the option for filling out the custom field is present when you edit a single address, if you try to do this for an address with available status, you'll get an error. If an address is available, you cannot track any information about it (otherwise we would consider it used from a licensing perspective). We have considered adding an option to create custom properties that are independent of the status, but this is not possible today.
I assume setting the subnet location at the subnet level is not an option? Where are your users seeing the IP that they wouldn't see the subnet?