So I ended up trying brewing the Christmas ale according to Mosher's recipe and made the 'potion' with the spices and liquors at the same time as brewing. I let the potion sit in the fridge while the beer was fermenting and then filtered the spices out of mixture using a coffee filter (this part was a PITA by the way).
I added just the prescribed amount of potion (250ml) to the bottling bucket and racked on top (which left about 400-500ml of 'spare' potion). A few weeks later, I have sampled a few of the bottles and they are only mildly carbonated.
A few take-aways:
- I probably should have added the potion on top of the beer after racking as opposed to racking on top. The bottling bucket had a strong smell of potion with some of the syrupy stuff at the bottom, so I'm not sure it mixed well with racking on top.
- The beer could probably use a little more potion (maybe in the neighborhood of 400ml total) or some priming sugar to get a bit more carbonation.
All-in-all the beer is really tasty and hella strong. It's only been bottle conditioning for a few weeks, so I think it's going to be even better in a month or so. It definitely fits with the story of "Wassail" accompanying the recipe in Mosher's book!