Not a schematic, but shows the flow:
BLEED SOURCE (R or L ENGINE)====BLEED AIR VALVE====PACK VALVE====CHECK VALVE====TRIM AIR PRESS MOD.VALVE AND SHUTOFF===MIX VALVE====DISTRIBUTION.
There are 3 TRIM AIR consumers:FLT DECK, FWD CABIN, REAR CABIN.
PACK VALVES are upstream of the TRIM AIR pressure modulating and shutoff valves. Pack bleed plumbing, while not entirely enclosed, is more or less sequestered within the pack machinery. Raw bleed-fed TRIM AIR plumbing, however, travels around the A/C BAY in order to get to all three post-mix manifold distribution ducts.
These 'outlying' areas are monitored by the WING BODY OVERHEAT sensing loops consisting of the L & R AC BAY loops, and further aft, the KEEL loops. Packs have their own dedicated overheat sensors.
While the above describes the flow and physical layout, I am not 100% positive that this is the whole story behind the reasoning for the procedure.