Your concerns seem ill-founded given that these are solved problems. There are working implementations of low-barrier cross-language compatibility, today, for both the JVM and the CLR.

Here's one example; Rhino provides pretty seamless bridging from JavaScript to Java:

https://developer.mozilla.org/en/Scripting_Java

I've used it to run user-supplied JS inside the JVM.

PHP is pretty easy too:

http://quercus.caucho.com/quercus-3.1/doc/quercus.xtp#JavaPH...

Scala is even more lightweight here:

http://www.codecommit.com/blog/java/interop-between-java-and...

Bear in mind that these are all for the JVM, which wasn't even designed with the intention of being a general-purpose multi-language VM.