I have never had, never seen, nor ever heard about anyone I know having an issue with their tobacco drying out in a jar. I would assume such a thing to be possible for someone who didn't make sure the jar was ever truly sealed, or someone who reused a jar lid (which you are supposed to throw away).
One week after canning, go through and remove all of your outer rings from the jars. I have been canning as my grandfather and great grandfather canned. Removing the outer ring from everything; tomatoes, beans, tobacco, whatever I have canned. Doing this, you ensure that there is a seal between the jar and the lid, because it will hold itself on. There is never a chance that a jar that I have sealed has compromised it's seal, because it would be blatantly obvious.
That said, I don't believe there to be a method of saving and aging tobaccos that surpasses the jar. I think that the mylar bag is ok, and maybe has it's great uses, but it doesn't surpass jars in any actual quality that a jar can provide.... except weight.