Here's what you do without leaving obvious chargers dangling out of outlets. You don't need to even send a guy in a maintenance uniform out to the site, or tamper with installed equipment.
You're a decently high-capacity Chinese factory that makes custom USB outlets. You make a "special" line with a zero-day chip or firmware inline with a cable. The cable only needs to be a little fatter to accommodate some unobtrusive electronics. They are slid under the insulation and there is no dedicated PCB that may attract scrutiny.
You wait until the order comes in for the site(s) you wish to target, and you ship them off.
The countermove to this, of course, is that the installer does a fuzz test of the charging station with a few common devices, trying to tickle the bug, and also a protocol analyzer that will inspect the USB data stream for anything out of the ordinary.
My armchair quarterback mind says that the above security testing should be fairly effective if you are dealing with a low-level adversary. A state-sponsored one with sufficiently large enough state would not be hindered by puny countermeasures like that, and would be able to target more accurately.
Here's another countermeasure on the consumer level: optocoupling. This is good to mitigate voltage and amperage damage, even accidental or unintentional types. I suppose it would prevent charging too, but there's got to be something useful about it.