I am not sure it is worth worrying about the oil draining back into the sump, as the amount of oil contained in a cooler for the Savage and the related plumbing is not going to be significant. I bet you won't have more than an extra pint in the whole cooling system. The only down side is that if the system does drain the oil out - until that oil is replaced......there is no oil flow to the cam. This should not be a problem as it will only take a few seconds for the refill, and the cam should have some oil in place from the last time it was run as long as you have not tipped the bike over on it's side and drained the oil out of the little recess beneath the cam lobes. This oil drain back situation occurs whenever our engine sits a long time even without the cooler, and the oil will drain from the passages in the cylinder, engine case and head and the oil pump has to refill those passages on start up (unless there is a check valve somewhere that I don't know about).
You can get check valves for oil flow. Triumph motorcycles have problems where the wet sump oil tank can drain completely down into the sump - so they install a check valve to prevent this from happening. It would probably work fine in a cooler installation, as the Triumph passes all the oil through the valve by gravity when the engine is running, and the Triumph most likely pumps a lot more oil than the Savage.
http://www.ebay.com/itm/Anti-drain-wet-sump-ball-check-valve-with-window-3-8-... It would allow you to mount the cooler wherever you want and you don't have to worry about anything flowing back down through the oil pump and into the sump. You really don't need to worry about draining the cooler and lines during oil changes - the little bit of oil will not contaminate the new oil much.
You also need to mount the cooler so that the flow enters the cooler at the bottom, and pushes the air out the top as it is filled with oil. If the oil enters the top - the air is not pushed out of the cooler and the oil will not flow properly, and the oil will just drip around the air bubbles and the cooler might restrict flow and it will not cool well.