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Currente Calamo | Frenzy

It is remotely possible that privateers may themselves be considered "enemy combatants" under the Geneva Convention framework.

They aren't straight-leg military, so the would exclude them from GC3 Art 4.1. They would then need to fall under GC3 Art 4.2, requiring:

--that they "[belong] to a Party to the conflict;
--[are] commanded by a person responsible for his subordinates;
--[have] a fixed distinctive sign recognizable at a distance;
--[carry] arms openly; and
--[conduct] their operations in accordance with the laws and customs of war.

The first four are pretty easily satisfied. The last one might be a problem, however. One might suggest that the use of letters of marque rises to the level of customary international law. This is problematic, however, given the general abandonment of the practice since the early nineteenth century. The same problem attends the customs of war argument. The counter-argument might also suggest that the implementation of the Geneva Conventions definitively superseded the prior laws and customs of war; thus, even if letters of marque were customary prior to the GCs, they would not be subsequently.

I'm not sure I'm particularly persuaded by any of these arguments. However, they are intriguing. More's the point, they would also provide political cover (however flimsy) to any party who might seek to detain American privateers indefinitely.