Breaking-in a Volt properly may be difficult to do since the driver does not have direct control of the throttle.
To properly break-in any engine it should be:
1. warmed up first, gently
2. driven fairly aggressively with frequent stops for about 20 miles.
3. get an oil and filter change
Since most people are terrified of pegging a new engine to full rpm the owner's manual wont tell you this. The factory has already ran the engine (in a dyno) to full power for a couple of minutes, to verify that it works properly and to do a partial break in.
The bore of the engine is rough, it has been scratched in a crosshatch pattern when it was manufactured.. these scratches serve two functions, to quickly wear-in the piston rings so they seat fully against the bore and to retain a film of oil to lubricate said rings. The roughness at the edge of the scratches is what wears-in the rings in the first 20 miles it has been driven.. once that roughness quickly wears off then the seating-in process will slow down or completely stop.. your new engine will never perform right if the rings have not fully seated.. it will consume more oil and it wont develop as much power.. I know it seems contradictory but after those first 20 miles the ring wear stops almost completely. You need to drive it hard since the high combustion pressures force the rings to expand hard against the bore, also you need to slow down often so that high crankcase vacuum can suck metal particles and chips out from behind the rings and down into the oil.. and then you do an oil/filter change to get rid of those chips.
This is how I would break-in a Volt:
1. get on a lightly traveled hwy, drive the car normally until the battery is depleted, about 40-60 miles.
2. once the genset starts up, continue driving normally until it warms up.. perhaps 5 minutes?
3. Lower all windows, turn on your headlights to high beams, crank up the AC (or heater) to maximum, set the car in Mountain Mode and start driving fast.. up to 80mph is fine, drive like that for a couple of minutes and then slow down to 45 for a few seconds then crank it up again. Dont drive it at a constant speed.. hopefully the computer will rev the genset up and down and that is what we want..
4. change the oil and filter.
Perhaps GM will go the extra mile and run the engine at the factory a bit more than normal. Note that the engine must be loaded while doing this.. just reving it up while parked wont do anything to break it in.