John,
If I understand what you are looking for, you might be able to implement this using the Outbound IPv4/IPv6 settings on the domains. After setting up the binding for the IP address, go to the Manage > Domains page and select the domain you want to set a specific outbound IP address for. In the Options card on the Options tab, there are settings for Outbound IPv4 and Outbound IPv6 which will include options for "Automatic" or any of the bound IP addresses.
Hope his helps!
Andrew Barker
Software Developer
SmarterTools Inc.
(877) 357-6278
www.smartertools.com