Well, according to the military appellate courts and law, a military member can consent to a search. However, is that realistic. The military is a society that follows orders. Can the circumstances surrounding the request for a consent search be so (objectively?) onerous as to make a refusal futile. I think so.
Remember how the suspect ends up being asked to consent to a search.
1. The COC tells the SM to be at the 1st Sergeant's/Shirt's office at a certain time.
2. Suspect shows up and is then escorted to CID, OSI, NCIS, CGIS. There is no choice, what person is going to tell their commander or supervisor they ain't going? It is futile to resist. Of course they play their don't tell him where you're going or why until absolutely necessary game. In my view this is "custodial." (note, I also raise this in a request for pretrial confinement credit, one day, for being in custody of the COC and law enforcement, and usually get the one day credit.) There is already a heavy influence of command on what happens next, despite appellate court opnions that all is "voluntary."
3. The suspect is then interviewed by law enforcement. Again, based on how and why the person is brought to law enforcement, any further interaction is custodial. As a custodial interrogation, Article 31(b), UCMJ, and Tempia apply. [n.1]
Here is a case in federal district court on how the civilian community looks at consent and futility to say no.
United States v. Moore, No.: 3:08-CR-60, 2009 U.S. Dist. LEXIS 5259 (E.D. Tenn. 23 January 2009):
determined on a case-by-case basis by considering the totality of the
circumstances-including the characteristics of the defendant and
circumstances surrounding the search. See Schneckloth, 412 U.S. at 226. "[C]onsent must be proved by clear and positive testimony, and, to be voluntary it must be unequivocal, specific, and intelligently given, uncontaminated by any duress or coercion." United States v. Scott, 578 F.2d 1186, 1188-89 (6th Cir. 1978)United States v. Worley, 193 F.3d 380, 386 (6th Cir. 1999). (citations omitted). Among the factors the Court should consider in
accessing the totality of the circumstances are "the age, intelligence,
and education of the individual; whether the individual understands the
right to refuse to consent; whether the individual understands his or
her constitutional rights; the length and nature of detention; and the
use of coercive or punishing conduct by the police."
n.1. United States v. Tempia, 16 U.S.C.M.A. 629, 635, 37 C.M.R. 249 (1967), incorporated the requirements of Miranda v. Arizona, 384 U.S. 436 (1966), into military practice.