Maybe we are over thinking your question a little. You seem focused on the socket itself.
The SOCKET can only have the voltage coming out of it that you feed to it. The voltage NUMBER that you might read on a socket is the MAXIMUM voltage that the device is RATED to handle. It has no effect on what comes out of it.
Here is your concern as I see it:
You want to put in a socket in the panel that can be used for YOUR your diagnostic equipment. That's fine, but what you put in as a socket is totally up to you if you want it to ONLY be used for that diagnostic equipment (although there may be other local rules and regulations to consider). But of more concern than the voltage rating of the socket itself, is the voltage input rating of the diagnostic equipment. If the nameplate says 240VAC at 50HZ, you may be in trouble if you feed it 220VAC at 60Hz. We cannot help you there, that is something that you must discuss with the supplier of the diagnostic equipment.
"Will work for salami"