If you can find some chocolate extract, then you can do test samples to figure out how much to add to make the batch taste how you want.
Take a small size sample of your base mead, measure the size of the sample, mark down how much extract it takes to make the sample taste good, and then scale up to find out how much extract to add to the main batch.
128oz per gal, so 5 gal = 640oz. If using a 4oz sample, multiply the amount of extract in the sample by 160 to know how much to use in the full batch.
Hope that helps.