2005 NEC 690.64(B)(2) allows an exception for a "dwelling unit". If I am making load side interconnection to the main distribution panel (MDP) of the PL&P service of a 180 unit apartment building, is the exception allowed. Note: the loads on the MDP are common area lighting, laundry, compactor, etc......no apartments.

Yeknom:
IMHO, I would say the tap is permissable as long as you comply with the rules stated within 690.64.

However, I suggest that you contact the local AHJ, and state your intentions, along with the pertanent information for your installation.

PLP = Public Lighting & Power
PLP service is power for the common areas in a building that are shared and under a separate meter paid for by the landlord, condo, or coop.

Yekmon:
As I said above, I see no issue with your backfeed connection within a "house" or 'landlord' (Common area) panel within a multi occupancy structure.

Run it by the local AHJ on your side of the river and see if he is good with it, or has a different opinion.