Cops are supposed to cite drivers for license plate covers and I see them out in NY the time--how can we expect them to enforce the display of their own numbers?
The only part of the uniform that police can't easily modify is the badge, which is also the part that unambiguously proves that they are police when necessary. (Not that most citizens would know how to authenticate the badge of their local police, much less those of all the federal LEAs [1]).
[1]: http://en.wikipedia.org/wiki/Category:Federal_law_enforcemen...
The headband of a uniform cap would also work well in addition to a name tape.
There should also be a requirement that police identify themselves by name/number verbally in interactions when it is practicable to do so; e.g. "You are under arrest, I'm Officer Johnson badge #3898390 of the NYPD, ..." Coupled with audio and video recording devices, it helps create a record.
"I want x to be subtle in the way that a shotgun to the face is subtle" i.e. not subtle at all.