I ran the flue of a 700 gallon Taylor water furnace into and out of a 55 gallon drum as a
transition from a horizontal to a vertical chimney.
I ran it for three years without incident and then took off the clamp and barrel end to clean the flue, the fly
ash build up was amazing!
There was probably at least 1/3 of the barrel filled with ash, on retrospect it probably was acting as an insulator and limiting the radiation from the barrel.
As far as draft, if the barrel is cold it will hinder draft but as long as the barrel is above ambient temperature it will assist. I would expect it to cool , and heat quickly as smoke is introduced.
Since most pellet stoves are forced draft I would think it would work fine, as a way of maximizing your exposure a baffle forcing the smoke through a twist or up and over a center divider may be useful.
+1 on the CO detector, it would be an awfully stupid way to perish when detectors are so cheap.