I am sure that this can be done (there must be cases where it is really necessary) - but it makes sense that it will be very complicated. As moondog says you will need to go through immigration again, so it may only be simple (I use that word with caution though ...) if you have a multi entry visa or residence permit or are a Chinese citizen. There may also be problems to re-enter China using TWOV as strictly speaking you will not have just arrived from another country in transit, though there may be some flexibility in the system here given that you will have the hexagonal exit/entry stamp in your passport (but I wouldn't bet on it ...).
Also just thinking about the geography at T3 this is not going to be simple. Access to the train from T3E to T3C (baggage area and exit) is only on the inner platforms, so just going back through the immigration area (where you arrive from T3C to T3E on one of the outer platforms) won't help as you can't get back on the train to the baggage reclaim area from here. Of course there there may be another manned security door to allow you to get to the inner platforms - I have no idea where this would be located though. In the case of an aborted take off at least the passengers will already be on the upper 'arrivals' level of the building, though that still leaves the immigration and visa issues.
I think the bottom line here is don't try and do this unless you are prepared to sit it out in the Air China lounge for the duration of your stopover. Of course if you want to try, please report back with your findings ....
