Yes, Skywalker would be treated as a potential war criminal, subject to trial by military tribunal.
The best way to see how someone like Luke Skywalker would be treated under U.S. law (buttressed by whatever international treaties the U.S. has signed) is to look at what happened to unaligned individuals who conducted similar attacks on large military installations and vessels. Unfortunately for this discussion, such attackers usually end up dead, either dying in a suicide attack or being killed by local police/military to end the attack.
Looking further up the hierarchy of the terrorist organizations behind such attacks, terrorist leaders are typically treated as unlawful enemy combatants. As an example, al-Qaeda claimed responsibility for the 2000 bombing attack of USS Cole. Under U.S. law, in particular the Military Commissions Act of 2006, members of al-Qaeda are classed as unlawful enemy combatants and are subject to trial under the military tribunal system that the MCA sets up. And so Abd al-Rahim al-Nashiri, the alleged mastermind of the Cole bombing is being tried under that system today.
So Luke Skywalker, an unaligned and therefore unlawful enemy combatant, would likely be subject to trial under military tribunal if apprehended. He would also very likely be subject extra-judicial killing thanks to the AUMF which is how U.S. force is currently authorized to assassinate terrorist leaders.
Some good news for Skywalker: waterboarding and other previously authorized methods of "aggressive interrogation" were prohibited early in the Obama presidency. I note with satisfaction that in this at least the U.S. once again treats prisoners better than Emperor Palpatine.