Battlecruisers for trade protection and cruiser hunting

Would battlecruisers still have a role in a treaty system built around new capital ships?

Yes, especially for trade protection and cruiser hunting, but most old battle-line ships would soon be outclassed.

Trade protection, interdiction, and cruiser hunting give Britain or Japan reasons to retain battlecruisers. Their speed could answer armed commerce raiders and threaten enemy cruisers while the main battle fleet handled heavily protected opponents. Only battlecruisers are expected to receive major modernization, mainly to preserve their speed relative to an emerging faster battle line. A continuing capital-ship programme would therefore make most pre-treaty battleships obsolete by the Second World War, with selective high-speed battlecruiser modernization the more plausible exception.