I had an open bottle of PX sherry, so I subbed that for the port, and decreased the syrup to compensate for the added sweetness. Although it obviously changed the character of the drink, it was still very tasty. The richness of the rum and wine were front and center, while the spice of the rye really came through on the swallow.
WOW. Had this tonight before dinner with some cheese and crackers. Perfect. Nicely sour, and the flavours of the rum, rye, and port blend wonderfully together. Thanks.