Mick, you're indeed quite right that Haddo's and Barbary Coast contain some additional flavoring, but as you guessed, they're real brandy and rum, not aromatic essences or artificial flavorings. Of course, they will leave a hint of their presence behind, but it smokes out quickly and easily. Actually, the perique will leave a more dominant ghost behind. I'm really talking about those tobaccos that are so heavily sauced that the tobacco almost becomes a secondary player.
Tokerpipes, I feel your plight. Some straight tobaccos are also hard to eject. If you're not a Latakia or perique fan, it can take some time, but you'll eventually conquer it if you keep at it. I assume you've done all the normal things - reaming, a good cleaning with alcohol. If you haven't tried the salt treatment, it might be worth a go. Put a pipe cleaner in the shank, fill the bowl with non-iodized salt, I use koshering salt, and moisten the salt well with the highest proof white alcohol you can get your hands on. Cheap vodka works very well. After a day or two, the salt will be brown and nasty looking. When it's dry, dump it out, use paper towels or tissues to get rid of any residue, and let the pipe sit for a couple days to thoroughly dry. It should make quite an improvement.