The ship class, I think, is mainly (not entirely?) for pricing and production. One thing that could be done is to set them as FREIGHTER, and make all freighters impossible to produce in the shipyard via Economy.gf. They would still get pulled in for missions. What about tagging them as "Q" so they appear in convoy raids/escorts as Q-ships?
I've never known what "L" does. So much for documentation, eh? You'd think we'd all have figured out these things by now.