Some micro chest rigs are capable to attach directly to backpacks’ straps with lilltle adapters. For example, Civilian Lab Ventrapack, 5.11 VTAC Chest Rig, Shellback Go Time Chest Rig, and others.
Speaking further about chest rigs attaching to backpack’s straps’, there is a temptation to attach them directly on user’s body AND drive straps throught the back, not using the neck strap. Only one obstacle is present – usual harnesses are made with four upper attachments for buckles, while those micro chest rigs have only two (some of them).
Now this is a solution – binoculars harness:
It has two attachments for upper buckles, lower buckles are still used for strap belt.
What is the point ot using such a configuration? You can use chest rig on the go with loaded pack on your back, and use it solely then bearing light load. And harness never interfere with pack’s straps.
Update with derivative concepts: