How would you say 'insigniaed BDUs'?
I would try to avoid saying it at all costs! BDUs (Battle Dress Uniforms), while commonly found in military fandoms, are an uncommon enough term that the reader may have to pause and remember what they are before figuring out what the insignia might be on. (I confess, when I first grabbed this question, I wondered for a second if the questioner meant
underwear with an insignia.)
There's also some redundancy in the statement -- a dress uniform, by definition, carries insignia. To make the statement easier to read and understand, it's probably easiest to separate the two concepts.
This was the first time Daniel had seen Carter in BDUs. He spent a long time studying the insignia on her collar.
"Can I help you?" she asked, with her eyebrows raised.
"Oh," Daniel said. "I was looking at --" He gestured. "Your neck. The insignia...."
Carter's face relaxed into a smile.
Note that even though "BDU" is grammatically correct, "BDUs" is the more common usage of the word-- perhaps because a uniform has more than one component.
If you do have to use a phrase like "insigniaed BDUs" -- such as, perhaps, "decorated BVDs" -- you should just follow the normal rules of grammar. For tips on when to abbreviate, check
chomiji's
excellent entry on the topic. (Another note: Remember that
BDUs have been phased out in every branch of the US military; check your canon and branch of service if you want to be accurate!)