This is a thread where I talk about how we did this: https://news.ycombinator.com/item?id=8332104
We used to use it a lot - a trivial Java webapp running in a slightly misconfigured Tomcat with a slightly misconfigured Apache in front on a slightly misconfigured Unix box - though it's fallen by the wayside because we never quite get around to the faff of setting up a slightly broken test system for them. So having that as a service would be pretty good.
If this is what you're doing: I suggest not just keystrokes, but that they keep a log of what they're doing and thinking. Because we never expect them to fix everything wrong with the box, but we do want to know how they approach it.