I can see this is an old thread but I made something quite like this recently. A liquorice porter, bittered with gentian (NOT hops). I waited until a very late stage before adding fresh mint leaves, which I think helped them impart more of their aroma and flavour than if I'd added them at the end of the boil. Spot of grated ginger also added to round out the flavour. I bottled it two days ago with some sugar for carbonation; it has a unique taste, with some really attractive notes - strong fruitiness from the mint and ginger, residual sweetness from the liquorice and malt, and a clean but not overwhelming bitterness from the gentian.
Will have to wait for a week or two to see how it tastes out of the fridge. I think it's quite promising.