The "special lane at customs" referred to above, is I believe actually the special International Transfer/Transit desk for immigration. If arriving on Air Canada, that would be Terminal 3 arrival at PEK, the special desks are just before the regular immigration desks. If transferring to MIAT, you almost certainly won't have your boarding pass so show the desk officer your confirmed onward itinerary (leaving less than 24 hours later). Then go forth and pick up bags and recheck with MIAT--if too early to recheck, take with you to hotel or (for daytime transits), stick temporarily in Left Luggage for a small fee. MIAT is also in T3.
I believe MIAT does not have interline agreements with too many other airlines, so there is a good chance when connecting to or from this airline to another, you'll be doing a manual baggage pickup and recheck for onward. Make sure for your other airline, you know what terminal it arrives to/leaves from.