Can the US military legally hold non-POWs for an indefinite period without recourse to a trial?

If so no problem on a legal level.

The issue becomes that Gitmo as it is referred to is a PR blackeye and eats away at the value of the moral high ground. If anything it makes the moral high ground look like a marketing blurb and the fine print is 'Warranty not valid outside of the following areas: USA'.

Equal rights is only true when it is equally applied to friend and foe. Integrity is when action and word is one. To do otherwise is mere marketing.

If your laws will not give you the ability to hold them in your territories, change the law, don't make a special zone. Be upfront with what you are doing. If you are doing the right thing then do it in the open. From the PR side it looks like a duck, quacks like a duck it must be a duck.