Hmm, I understand, sir. It is impossible without breaking EULA.
Not really -- you can create an approximation of the RGSSx hidden classes which simply respond to the messages sent from the Ruby side. Getting the methods which these objects respond to is simple, and it really isn't all that hard to write wrappers around, say, OpenGL which provide the same general functionality. There are a few problematic classes, of course, but for the most part, it's not too difficult to do.
Also, this may not be exactly what you're looking for, but Enelvon and I wrote a REPL for Ace (SES Console) which allows you to write code and dynamically test it inside of a running Ace game being played in test mode. This isn't
exactly a sandbox, but you can use the macro functionality that we provide to write an externally defined macro containing the code you want to test, dynamically call that macro during play testing (which loads it into the current game), then tweak the code as the game is running and get the basic results that you want.
The REPL is available
here. The latest stable version is 1.3, but we also have an unstable version 1.4 available from the unstable branch
here (which is actually very stable and contains a lot of functionality missing from stable version 1.3).
If you need more instruction on how to get the SES Console working how you want it, please feel free to send me a PM. I'd be happy to help.