First off, most of this group/forum doesn't really get too involved in hybrid setups they are more the pure EV type...
Secondly, its been discussed before and is possible. A similar setup is the EMIS that Netgain sells, of course EMIS is for a rwd car, but the concept is the same and it has been shown to work.
A few things to keep in mind...
The axle will be a fixed gear ratio, so you have to pick a ratio that will accommodate the maximum rpm the electric motor can take...for example, if you choose the AC50, it has a max safe rpm of say 6000rpm, you want to make sure that your max speed on the HHR doesn't exceed 6000rpm on the electric motor. What you will find is that the torque you expect wont be as much as you would like when you have to gear the motor for speed. Now, you can get away from this problem by (a) creating sometype of clutch that would release the motor from the shaft, it would be better if it was an automatically actuated clutch so less human error, or (b) attach the motor to transverse transmission and electrically control it to shift at a certain rpm/speed, the shift method would actually provide you more power through the speed range where as the clutch method could give you the rowdy AWD launch but would just be dead weight once disengaged.
Weight is key, you don't want to add more weight that the power you are gaining, every 100lbs needs an additional 10hp... you should look into the lithium polymer batteries that are commonly used in RC cars but that are also used in one of the fastest EVs around, John Metric's Fiero, his cells are 45C continuous 90C peak, so they can pack a punch in a small volume.
consider an unconventional method of AWD/Hybrid...check out the EVpropulsion.com's motorcycle wheel motors...http://www.ev-propulsion.com/motorcycle-hub-motors.html
30kw (40hp) peak. If you were to put one on each wheel that's +80hp, if you were to go with the dual motor/wheel, then that's +160hp
It would take some custom machining to a lightweight racing rim and would require you to bolt the torque bar to your car, but it would be one of the only removable hybrid setups on the road...unbolt the torque arm, unplug the wiring, and replace the rim...done. Note: I am not sure of the weight of the wheel motor, however if it is to be used on the drag strip or generally smooth roads, the additional unsprung weight will matter less.