This does not answer your specific question for PamguardPAMGuard, but there is a ship detector for Matlab-based program Triton. The detector runs on Long-term spectral averages, which can be created in Triton. I have not used it personally, but may be worth exploring.
You can find more information about the detector in the GitHub wiki here: https://github.com/MarineBioAcousticsRC/Triton/wiki/Ship-Detector
And you can download the latest version of Triton here: https://github.com/MarineBioAcousticsRC/Triton