Rec for bike backpack with side zipper into main compartment
Most of the nicer courier bag/backpack companies don't do a zipper down the length of the bag because that creates a weak point for not only loss of contents, but water entry. Some will do laptop pouches, but if you're looking for a heavy duty courier bag it's slim pickings for external zippers into main compartments.

The closest I came to finding something like what you're after is from either Crumpler that have some laptop sleeves and/or zippers to main compartments or Freight Baggage that have a smaller secondary compartment that is easily accessible. I looked at RELoad, Freight, Seagull, Bailey Works, Trash Bags, Banjo Brothers and some other smaller companies.