Unless I'm missing something, since the OP is flying from an international location without pre-clearance, he or she will have to pick up checked luggage at MIA anyway. I would suggest asking the agent at the baggage recheck desk or taking your bags to the general check-in desk and asking there. I have had luck with this before (had a LIR-MIA-LAX(overnight)-SFO ticket and upon arrival was able to get the check-in agent to put me on MIA-SFO direct).