I'm not a law-man, but my laypersons understanding of the law against impersonating a police officer hinges on intent. If you intend to deceive someone into thinking you're a LEO, and that can be proven (or can't be disproved beyond a reasonable doubt) it's illegal. Doesn't matter if you're in a suit trying to convince someone you're Special Agent Mulder, or if you're in body armor and a mask trying to convince someone you're ICE.

Also as an American I 100% agree with magikmw's point, that illegal or not it doesn't matter if ICE decides they don't like it - until you can manage to get in front of a judge, which may not be guaranteed. Then again even if you do, it has to be something you can reliably prove.