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>In my application it would be more convenient if the userdata was an int, 
>rather than the address of a memory location.

Use lua_newuserdata to allocate sizeof(int) bytes and store your int there.

>However could the current 
>lua interpreter, or a lua program running in the interpreter, ever perform an 
>action (such as dereference, free, realloc, etc...) on a userdata with the 
>expectation that it is a pointer to a valid memory location?

No. Lua never looks inside userdata.

>Secondly, if it is safe to provide userdata values which are not pointers to a 
>valid area of memory, then is this safety guaranteed by the specification, so 
>I can likely expect this to work with future versions.

I think that ANSI C says (or implies) that implementations are free to flag
invalid pointers, even if they are never dereferenced.