What is the procedure to perform sandhyavandanam at temples? Is it done facing East or North directions? Or is it done facing the main deity within the main sanctum? (Note: Remember reading somewhere that since every direction is a vastram of Perumal, it is sufficient that sandhyavandanam is done facing the main deity).

While performing sandhyavandanam at temples, it can be done facing the main deity. At other places, it is done facing either East, North, or West as appropriate.
Since Perumal is present in temples, it is the practice of elders to consider that Perumal Himself is verily present and perform sandhyavandanam at the temple facing Perumal’s sanctum.