If you control the compiler and the language is memory-safe, you get the same guarantees with JIT-compilation that you do with interpretation. All execution runs within the memory boundaries specifically allocated by the runtime system, and any other interaction with the machine must go through the runtime libraries, where you can perform any checks you want.
I don't see this kind of sandboxing as likely to help. Whether your scripting language is compiled or interpreted, if any accessible portion of your system is written in a memory-unsafe language like C, that is where crackers will find the most damaging exploits. E.g., http://stackoverflow.com/questions/381171/help-me-understand... which attacks a native XML toolset (some "data binding" IE feature I don't understand) without breaking any javascript rules.
