A big quest ...
Don't know about USB to RS232 but it is a good question. Another idea is to check, whether there is something like a USB compatiblity mode over the EFI console (the old REFIT had it) - or whether it is a driver problem.
I have here a MacBook Pro 9,1 from mid 2012 but I never tried OpenBSD on it. I tried FreeBSD on it but the developer doesn't update asmc so I gave this up. Maybe I should try it ...
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