[signaturefish]

To answer your question, the lifting technique I refer to works well on flat paper and variably well on non-flat paper. Something as delicate as a letterlock, I'd be seriously concerned about my hot knife nicking or tearing a part of the lock that I can't see under the seal ... and /that/ would be obvious.

So it's not so much that the seal holds the lock closed, as that the seal obscures the lock to the point that opening non-destructively is much harder.