iOS / AudioSessions: Route audio IN through the INTERNAL MIC when EXTERNAL headset mic is present?

Question

As it says on the can really:

Can I route audio IN through the INTERNAL MIC when an EXTERNAL headset mic is present?

If so, how?

EDIT:

I found this:

http://developer.apple.com/library/ios/documentation/AudioToolbox/Reference/AudioSessionServicesReference/AudioSessionServicesReference.pdf

p.18

so overriding to use the built-in speaker AUTOMATICALLY switches to using the internal microphone.

But I can't see any way to do it explicitly.