Personally i think i would like the Secondary idea over the Boil or tincture idea. Just seems like it would give you more of the pronounced aroma and flavour. the next question is how much before it becomes too much!
I have done secondary using a stuffed tea ball full of fresh mint. I change it every 2-3 days and take a small sample to gauge when to stop. Last time the flavor held well for about 6 weeks after carbonation, then it fell off quickly. I always try to keep a couple of beer to test how it ages. This one doesn't hold up well for that.
What else do you flavour the , traditionally I know there is chocolate notes in a porter, do you try to make that more pronounced with more chocolate malt or cocoa nibs or does the mint usually hold its own without adding an extra complementary flavour???
I have just used a standard robust porter kit with mint leaves. I did a coffee porter a couple of winters ago where I crushed coffee beans and put them in tea ball during secondary