Brake hanger causing seatpost slippage?
2.8Nm for a collar on a steel/aluminum post is incredibly small. My carbon frame bike (with aluminum seatpost) specifies 5nm and 5-7 is a common value (Refer park tool website).  I probably run around 2Nm, as I use carbon paste and have it no tighter than needed to stay put, but would happily go higher if needed.  

First have you cleaned and greased the threads of the clamp bolt. This makes a big difference to the clamping force at a given torque.  If you want to stay at 2.8nm, try carbon paste. Its designed to increase friction and reduce required clamping forces on carbon, but works well on metals. 

Without suggesting you become "A gorilla with a spanner"  - You could "over torque" the clamp (or replace the clamp that has a larger number printed on it). I can't imagine crushing a steel frame/aluminum post at any reasonable torque you can get onto the bolt size a seat post clamp has. I would have a guess many seat posts on steel bikes are clamped at 10 times or more that value with the only problem being getting them undone when needed.