Not a solution to your problem really, but when using pjsip, you are no longer limited to using one external IP for your endpoints/trunks. You can have more.
You can e.g. register trunks to your voice service providers out on one internet connection and use another internet connection for other trunks.
Provided you have different destination addresses.
Asterisk will put the correct external IP address, the one you have defined for each endpoing, in the SIP headers.
This is not possible in chan_sip, there you can only have one external IP address.