I think Steve's approach is the best approach.
Look at the volumes going into the fermentation vessel on your five versus ten gallon system and scale up based upon those volumes. For example, if your five gallon recipe yields 4.75 gal in the fermentor with 0.25 gal lost to the kettle and your ten gallon batch yields 9.75 gal then you have a 5% loss on the five gallon batch and 2.5% loss on the ten gallon batch. You gain a whole 0.25 gal efficiency on the ten gallon recipe which you should account for as a 5% increase in post-boil volume on the five gallon batch when scaling up. So you would want to take 95% of the spice addition in the five gallon batch and double that to 1.9 times the addition in your five gallon recipe.
Straight doubling can be an easy shorthand that will get you in the ballpark and maybe even have an undetectable difference from the five gallon recipe but with particularly potent spices I would be more cautious. With some spices a gram here or there can be the difference between balanced and overwhelming.