I'm not a lawyer, but under the law as it's written, I see two problems:
17 USC 121 allows "authorized entities" to make and publish accessible copies of works. An "authorized entity" is defined as
a nonprofit organization or a governmental agency that has a primary mission to provide specialized services relating to training, education, or adaptive reading or information access needs of blind or other persons with disabilities.
So if you, as a private citizen, decide to do this, it could conceivably be copyright infringement. You might have to set up some kind of non-profit organization to make it legal. It's also OK (I think) if you make such copies for your own personal use, so long as you don't redistribute them.
So far as I can tell, nothing under 17 USC 121 requires the original publisher to provide an "authorized entity" with a copy in any particular format (PDF, paper, or otherwise) for making accessible copies.
Basically, the law seems to have envisioned organizations of sighted people purchasing paper copies, transcribing them, and republishing them; not blind individuals doing electronic transcription for themselves.
It might still be worth contacting Hal Leonard and asking what they can do for you, but unfortunately it doesn't look like the law requires them to do anything for you. As Nij points out in the comments, this really seems to be a question about the company's policy, rather than the law.