Yes it would be possible to use bluetooth 2 with a car's handsfree system. It's possible but it's not worth the sacrifices you make on the device side for a small benefit. It would be sort of like using a floppy disk to transfer word a word document. The point of bluetooth 4 is that with low power consumption you can have devices connect and transfer info that you wouldn't have before because it would have used too much energy and not been worth it. While carwings could perform the same function, the privacy implications are probably what prevented nissan from adding it as a feature.
OK. We must be talking about two different things. Since I consider the handsfree the whole reason for having BT on my phone in the car, I don't consider having the Handsfree Profile active 'a small benefit'. In fact it's the sole reason I use BT on my phone: for safer driving if someone calls me.
Given that one is only going to be in the car for an hour, having a BT Handsfree connection is virtually free on just about every phone (remember BT power is nothing compared to a 3G or Wifi radio), so the 'sacrifices on the device' are negligible maintaining the handsfree profile (streaming is a different story, but it is still small compared to a 3G phone call). The BT radio power consumption peaks when the profile is active (talking on the phone and for the periodic battery & connection info that you see on the screen) and is the same for BT4 or BT2. The BT4 benefit is when there is no data being exchanged as part of the profile and only keep alive packets are being sent to keep the pairing active. Here BT4.0 maintains the pairing for almost free, however since BT2.0 uses very little power to retain the pairing, the difference between the two is only measurable when I stay paired with something for many hours (which is not the case for a LEAF pairing).
So if you're not using the BT Handsfree, well yeah BT4.0 would definitely have an advantage since the BT Profile is never actively transferring data and BT4's low-power benefit can be taken advantage of the entire time the pairing is up. However if you are using Handsfree, BT4 & BT2 power usage will be virtually the same over the hour or so you're paired and Gary's idea gives you the same functionality without the separate USB dongle.
I just tried the handsfree model with the Android simulator and it worked fine: My Car finder app is launched when the BT any pairing is lost, I check which one and if it's the one I care about I save the location. When my App is launched standalone I can show that last location on Google Maps (which means if I was energetic I could write code for some sort of active direction finder to find the car). As noted earlier on this thread, the tricky part is making sure location services actually has a good current location, I know this can be hit or miss depending on the phone. I'll contact a buddy who's got the Apple SDK to see if he can confirm that I'm correct about the pairing dropped notification not being available in normal BT2 iOS.
If it looks like it can be done on an Apple and my buddy wants to do it, I'll be sure he gives Gary a royalty