I eventually went with a recipe from Randy Moshe's book, although it didn't come out anywhere near the strength he suggested (5% rather than 6.5%). I also scaled it down to a 13-pint batch seeing as it was all slightly experimental.
It's half and half pale and wheat DME 500g each), plus a mini-mash of pilsner malt (190g), melanoidin mal (370g)t, oats and carafa II (100g each). A small amount of Northern Brewer hops (8g) were used for the whole boil, plus 5g ground coriander, 1g star anise and 3g of licorice root, with a Belgian ale yeast.
So, one month on, it's carbed up pretty well, pours a viscous but clear near black and tastes pretty good. The yeast takes some getting used to - its coconutty, banana esters are right in your face - and the next most dominant flavour is the licorice. It's overall well balanced, though I'd maybe like a slightly sweeter, more floral note, and more hops wouldn't hurt (perhaps dry hopping with Amarillo for aroma). I'd also be very interested in trying the same recipe with a less aggressive yeast, as I don't remember Haxan tasting quite so "Belgianny". It was more like alcoholic dandelion and burdock.
Has anyone else tried this style or got any comments on my effort?