A few years ago, when searching the internet for information on some Trader Vic's recipes (particularly the Tiki Puka Puka), I ran across an article that provided a recipe that they thought was a close approximation of the Trader Vic's Navy Grog mix. In short order I mixed up a batch, used it in a Tiki Puka Puka (a recipe from Trader Vic's Tiki Party that calls specifically for Navy Grog mix) and damned if it didn't closely resemble the restaurant version. I bookmarked the site, added the recipe to my notes, and have made it ever since.
One of the recipes I've used it in is the "Brake 3" that I'm serving on the tent crawl this weekend as Mojave Oasis. I'll post the drink recipe here after Mojave Oasis, but for now, here's the spice syrup recipe. Also check out the original post on Sloshed. It's a cool story that links back to Tiki Central.
Crack together: 3 cinnamon sticks, 2 (ok more like 4) whole cloves, 1/4 tsp cardamom (if you're starting from whole cardamom pods, use 1 tsp. pods), and 2 tablespoons whole allspice. Boil in 3 cups of water until 1.5 cups remain. Strain, and add 1.5 cups of sugar. Heat over low heat until the sugar is dissolved. Cool and bottle. Add ~1 oz. white rum to stabilize.
Navy Grog mix
1/3 cup spice syrup
1/3 cup fresh lime juice
1.5 oz grapefruit juice