Slowing a battleship does not make it a treaty auxiliary

Could limiting a battleship to 20 knots allow it to pass through the treaty system as an auxiliary?

No; its armament, displacement, and other warship characteristics still count.

Limiting a ship's engines to 20 knots does not settle its treaty classification. Gun size and displacement remain part of the classification. Those characteristics can still identify the vessel as a warship. The proposed speed limit therefore fails to create an auxiliary exemption on its own.