Your initial instinct is totally correct. 5 radio mics into a multitrack recording system will net you the BEST audio recordings, hands down.
As to the briefly costumed character (pun intended) this isn’t that odd. The singer Sam Harris did a lengthy tour in Joseph and the Amazing Technicolor Dreamcoat and had a whole segment clad in nothing but a brief loincloth.
I’ll pass along the technique described to me by one of the Sennheiser guys at NAB one year. Mic gets hair placement. Smallest possible transmitter placed in his briefs in a sewed-in pouch that allows for as much movement and comfort as possible. (costuming has to be on board, since you’ll likely need to make compromises between aesthetics and practicality. Experienced actors understand this and seldom give it a second thought since the point is to deliver their performance to the audience as well as possible.) Run the wire down the back adjacent to the spine, Use Moleskin to cover the wire – and then makeup to blend it into the skin tones.
Particularly with tight briefs it’s not easy – but it IS possible.
Good luck.
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