The question would then become whether or not the person would be subject to UCMJ jurisdiction. Which is not even remotely close to a simple question to answer. Admittedly, I am not intimately familiar with the UCMJ. But from casually browsing the text, I don't see any provision like this. They don't convene a general court martial in the same manner that a person gets indicted in federal court for criminal charges. It is like a commanding officer referral. And appears to require initiating proceedings within 120 days. Of course, there is likely a procedure for when the accused's whereabouts is unknown. But I doubt there is a procedure for when it would be merely inconvenient to abide by the 120 days. There are other officials capable of initiating courts martial, and the President is one of them. Still governed by the same procedures outlined by Congress though.

There are obvious cases where a person is subject to the UCMJ. And these are current + retired servicemembers. Remember that the UCMJ is a creation by Congress. A statute, if you will. And statutes are unenforceable to the extent that they conflict with the Constitution.

Fifth Amendment:

"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation."

Now plainly, as long standing case law interpretation, "arising in the land or naval forces" means arising out of the service branches. This does not necessarily mean a military offense; servicemembers subject to UCMJ can be tried for civilian offenses (dui, domestic violence off base, homicide off base, etc) by court martial. But when you are talking US citizens taken into custody on US soil...now you are governed by the rights in the 5th amendment. I think it would be difficult to imagine a scenario where a party was aprehended on US soil, is a US citizen, NOT a service member or prior service member, and is subjected to UCMJ.

If said person flees overseas, that is a different subject entirely. But once repatriated to the United States, those charges are in federal courts. If one wished to avoid this scenario, then a tribunal should be constituted for the accused to be tried without ever bringing the accused back to the US. This has actually been held to be Constitutional. Albeit, with materially different facts that would impact this analysis when applied to the current circumstances.

I don't see a way for UCMJ to govern unless the person was aprehended and then subsequently tried overseas and the crime was one punishable under the UCMJ. Or the person accused is not a US citizen and the same criteria of the crime charged being punishable under the UCMJ. That is quite a limitation.