I have made a few "pseudo" ports, or at least distant cousins of ports, when I had meads that finished too sweet. Hit them with a relevant ardent spirit and they become rather interesting instead of flabby and poorly fermented. Nearly a gallon of Speyburn 10 year went into a batch of too-sweet sack mead, for a sort of Anglo-Saxon variant of port, and I also fortified with bourbon a stout-style braggot that likewise was too sweet. If apple brandy was cheaper, an apple cyser mead fortified with the stuff would be a great little Norman (or west country) take on port...
I gather though that the fortifying agent, while technically brandy, is basically unaged, clear distillate, not a fine aged brandy or anything like that, so the above methods are not strongly analogous to port making.