Confused by your statement. If one uses a bottle from another vehicle (which I assume you imply a bottle that uses a regular radiator type cap), and use a secondary overflow bottle (connected to the outlet of the first bottle), isn't that essentially converting to an open system?
If so, then if one caps off the overflow outlet of the first bottle (with the regular radiator cap), then does one still maintain a closed system, and if so, isn't the OEM post, inside the OEM bottle, lost and isn't it )the post) an important feature of the OEM bottle to maintain the correct coolant level?
-Maz