Some caveats here...

First, it's worth noting that it's not a given that you will be able to get your return boarding pass before leaving the USA. It all depends on finding an agent at a U.S. station who is willing to try to issue the MGA-MIA boarding pass. IME, about 50% are willing to try, 50% insist that it's not possible for flights originating internationally because of security requirements. Of course, we all know that it's possible, if they know the system well enough, but some agents won't try.



Second, there's also a chance that your resourcefulness could backfire during these runs. One of my FTer friends got in HUGE trouble earlier this year with the security personnel at MAD for having checked in and received his MAD-DFW boarding pass the night before at DFW. He did it so he could stay at T4S and bypass immigration/customs/train, and go directly to the lounge to await his return flight. But, the AA security staff claimed he was trying to circumvent the baggage security questions, and was not following the correct airport procedures. It was a big headache and he greatly regretted doing it. Since I heard that story, I've stopped trying to get my return boarding passes from overseas quick-turn MRs in advance. It's just not worth the potential problems.