There's no explicit API to pin objects from the C side. Ok, so there's an implicit contract on which pointers to internal VM objects are still valid, once the C side has gotten them (const char *lua_tostring() is the biggest offender). But this contract a) does not lead to a practical implementation and b) is violated by plenty of C modules for Lua, because Lua didn't have a moving GC, so nobody bothered to follow it.

Now ... the interesting objects, that benefit most from the defragmentation effects of a moving GC, are the variable-sized objects. Sadly, userdata cannot be moved at all and strings are the only other interesting object type. Well ... see above.

[Table objects are not variable-size. Only their array and hash parts are. But these are singly-linked from the table object and can easily be moved around, anyway. The addresses of these memory blocks are never exposed by the VM.]