I didn't install my baro damper following this design.. My AA 260 uses 6" flue pipe, and I had a 6" baro damper.. so I installed it without the recommended design.. and I do ocasionally get a small puff of ash out the damper when the fan starts.. BUT, my boiler is in an outbuilding, where dust and ash are not an issue..
I'm pretty sure you could buy a 6"-to-5" adapter, and cut the crimp off of the 5" pipe and slide it throught he adapter so the 5" pipe ends in the middle of the damper's TEE. you can then seal the slip joint with RTV sealant or high temp tape..
The small pipe within a larger pipe with an inlet on the side of the larger pipe is a very common type of 'pump'.. air craft use this to move fuel within tanks,, it is usually called a 'motive-flow' pump.. it has no moving parts, and functions only with fuel moving through the smaller higher pressure pipe.
Spray guns, siphon guns work this way too.