The VPN feature on the phone is great and I use it often. I think this would be a useful enhancement.
Digium phones have a feature called Phone Networks.
Phone Networks determine how a Digium Phone should behave, depending on the IP address and netmask of the Phone as it sits on the network. If the Phone cannot get on the network because it is trying in vain to use the wrong Phone Networks information, use the Phone's Select Network option to select the Phone Network that matches the network you want the phone to use.
Basically the phone looks at its' IP address and can register differently depending on the address.
A use case would be having a phone that can be used on the corporate network and VPN. If the phone sees that it is on the corporate network then is does use the VPN to register. If the phone is not on the corporate network it fires up the VPN for connectivity.