How do I send Bluetooth audio from the Stereo (A2DP) to Headset (HSP) profile? I have a stereo Bluetooth headset. When paired and connected, both the Phone and Media profiles are supported.

I can hear Google Maps Navigation voice directions in the headset.

However, if I uncheck the Media profile (I am using the the media profile to connect to an iPod touch), then the voice directions come out of the speaker. I want them to come over the headset.

From this, I assume that the phone wants to send the voice directions to the A2DP if the headset supports it, and to the speaker if the headset does not support A2DP.

Is there any way to configure the Phone (Bell's Samsung Galaxy S) so the voice turn-by-turn directions are sent to the HSP (headset) profile (where to audio for cell phones voice calls is directed)?

Although I haven't tried it, I assume this problem exists for simple non-media headsets that only support HSP.
A2DP (media profile) is a protocol for high-quality unidirectional audio over Bluetooth. HSP (phone profile) is a protocol for bidirectional lagless communication (audio) over Bluetooth.

A modern headset typically supports both. A2DP is meant to play (stereo) music. In Android, whatever the system considers as 'media' (you can find this terminology in the volume settings), will be using A2DP. Whatever is voice ('ringtone'), will be played over HSP. I suppose all that likely will be is a phone call. 

The reason for this choice is that voice in a phone call is bidirectional and you don't want any delays on the line. Hence the lower (and mono) quality of HSP. In voice, quality is of less importance. However, the voice directions (which is not the same as 'voice') of your GPS are not a call, they are not bidirectional and delay does not matter, and they are played as 'media' in the system. Therefore they will be played using A2DP, or when you disable the 'media' profile in your Bluetooth settings, over the speaker. In summary, check your volume controls, they work similarly and will hint you what is what.

It is currently not possible to choose where audio is being sent, neither is that intended in both the design of Bluetooth or of any OS for the above reasons.