LIR_ref only supports register-based args at the moment. this means only the first two parameters (on x86) or four (arm) can be multiname or box references. we even have extra forth code to ROT arguments to fit this model, which is really unnecessary.
This may or may not be worth doing... ROT/SWAP/ETC becomes "free" inside superwords, so if we can constrain the few places that need this to inside superwords the penalty pretty much goes away. Might be easier to superwordify the few places that need it rather than add more general code for LIR_ref.