I'm not 100% certain I understand what you're referring to.
If you mean the system console is on serial port A (which is the default when no keyboard is attached), then no. The location of the system console is managed by the OBP and can only be changed during a reboot/reset.
It is possible that you could send output of the system to serial port b (enable a terminal, or mirror console output with consadm), but you could never use that port to abort the system until the OBP moved the console over there.
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Darren