exchange 2013 smtp connector routing question

Environment:

Exchange 2013 CU10 combined role server. Currently ALL external messages are routed through a 3rd party service provider using a smarthost in the send connector.

Problem:

Need the ability to send SOME external messages though another service provider.

Ideally I want to end up with 2 send connectors where 1st send connector is always used, unless user specifies in the subject or body of the message a key word which would instruct exchange to route through the 2nd connector. Wondering if this is doable with a SetRoutingOverride method?

Thanks