To Ottavio Caruso <ottavio2006-usenet2012 () yahoo ! com>: It definitely answers the question on how to define com0 as the default console after installation (the /etc/ttys part was specially helpful) and confirms the need to disable vga generic driver. Thank you.
To Ian Darwin <ian () darwinsys ! com>: I assume that means the software would accept it. But still, as far as I know, it is not recommended from an electrical (and electronic?) point of view. A physical failure in these computers would not be acceptable, I would rather not take the risk. Thank you both for your answers.