Hi,
metalua grammars are easy to extend as long as your extensions respect Lua's spirit. That's a feature not a bug: if your grammar extension is hard to express, it probably doesn't respect Lua's implicit rule that a single token lookahead is enough to parse it. It's also very likely that it introduces unforeseen ambiguities.
Semantics is easy as long as you can express your extensions as a function modifying ASTs.
Anyway, adding rules to statement and/or expression parsers is easy, especially if they are introduced by dedicated keywords, as is the case in most of Lua. Can you share an example of the extensions you're considering?