The term "Bluetooth headset" is very broad. Different headsets support different [Bluetooth profiles][1].

The most common profile is HSP (used mainly for phone calls), which seems to be independent of the headphones jack you are referring to. So pairing your Android device with a cheap headset that supports HSP only (e.g. Plantronics 242) won't affect audio routing to the headphones jack at all. Your Bluetooth headset and your Android device will continue to operate in parallel, but each handling a different audio stream (channel).

If you are looking to disable the microphone only in your Android phone, it seems that only a headset with an A2DP profile could be a possible solution. You may want to read at the following thread: http://android.stackexchange.com/questions/2819/sending-bluetooth-audio-from-the-stereo-a2dp-to-headset-hsp-profile



  [1]: http://en.wikipedia.org/wiki/Bluetooth_profile#Headset_Profile_.28HSP.29