I'd strongly advice not to place the Pi and key together. Rather make a hole in the wall and place the Pi and power supply inside the datacenter.
This will not only give you a lot more options for choosing case(s), it's also far less prone for abuse. Once inside the datacenter, people probably have better things to do than to 'pick' your lock circuitry.
Be sure to put the Pi and lock mechanism on the datacenter UPS. It might sound obviously and stupid that I mention this, but really, as we speak, people are getting denied access to server rooms around the globe during powercut if-we-dont-shutdown-that-box-properly-were-screwed situations.