Practical limits on a disguised merchant minelayer

Could a disabled-looking liner and tug secretly lay delayed mines while entering a harbor?

It could create severe disruption, but local harbor tugs and normal port procedures would make the accompanying tug difficult to control or disguise.

An oceangoing tug might accompany a disabled ship, yet harbor authorities normally send local tugs with the knowledge and maneuverability needed inside port. A moon pool could conceal mine release from the merchant hull, but every extra vessel and unexplained movement adds opportunities for the deception to be detected.