Will your circuitry be intrisically safe (IS), or will it be housed in an explosion proof enclosure? If your product is IS, then you can use any old keyboard.

If your circuitry is inside an explosion proof enclosure, then I assume you are looking for a solution where the keyswitches are on the inside, but actuated from the outside. The capacitive touch switches are promising as they work through 6mm glass. I assume the product will need to display a reading, so you will have a glass window. A final solution is to have an energy limiting barrier in the enclosure, which feeds the keyboard outside the enclosure.