Yes, it should help if the exhaust valve is closed long before the piston gets to the end of the exhaust stroke. In fact, if the engine is strong enough, it may be possible to cancel the first exhaust stroke and turn it into a high pressure compression stroke. It would be smart to use some kind of heat barrier coating in the combustion chamber to avoid too much heat loss.
http://www.hotrod.com/techarticles/e...ech/index.html
Another possibility would be to route the exhaust gases from the exhaust manifold into an expansion engine (like a steam engine) while adding some water.
To respond to NoCO2, yes, I think it makes sense to put the gases into a tank during the exhaust stroke. To get the maximum benefit from the pressurized gas, you would need at least two expansion strokes for every power stroke because the volume of exhaust gas at atmospheric pressure is more than twice that of the intake fuel mixture because of the higher temperature. Using two expansion strokes would turn it into an eight stroke engine if using one cylinder to do everything. Valve control would be complicated but definitely possible. It might be simpler to use separate cylinders to use just for expanding the exhaust gases with steam. They would operate on a two stroke cycle, just intake/expansion and exhaust.
I don't think a spark could ignite any more fuel left in the exhaust gas.