Equal accessibility is important to me, so I would want my book available in large print, braille, and/or audio-book formats, so it would be accessible to people w/ poor or no vision or w/ other conditions that would limit their ability to read a standard-printed book .
I know these formats can be expensive, and certainly many books are unavailable any of these ways. My question is, considering that this would be my first published book, and that I therefore have no clout at all, can I actually make this demand? Or would the publisher simply skip me for someone whose first book would be much cheaper to publish?
Should I mention I'd be willing to compromise elsewhere as needed, such as a smaller advance or giving up other rights, to make this happen?
And at what point in the query process do I tell my would-be agent how important this is to me?
The author is the seller of rights, and the publishers who acquire audio/large print rights etc. are the buyers. You can't actually force someone to buy something, (Oh how I wish you could!) even if you reduce the price to zero (your reference to smaller advance/other rights.)
Audio is the most likely subsidiary right to be licensed. Often the print publisher will acquire audio rights as part of their initial deal. If they do not, your agent can pitch them to an audio publisher. Whether an audio publisher is interested is entirely up to them. Even if it's hugely important to the author.
Large print rights are much more difficult to license. Large print publishers usually seek titles that are best sellers, and from established brand name authors. Large print rights are also often acquired by the publisher making the initial print deal. Of all the books I've sold, fewer than 10% have been licensed to large print publishers either thorough the initial print publisher or in a direct deal.
As for Braille, those rights are also addressed in the print contract. Most often they are made available at no charge to publishers who want them. It's a pretty standard clause.
That said, I've never seen a Braille book. But then again, maybe I'm just not looking in the right place.
I commend your enthusiasm for making your book available to people with reading challenges. It contrasts quite favorably to the fellow who was peeved that Braille rights were being given away. ("I wrote it why shouldn't I get paid" was his position. He did not sign the contract and I was happy to sever my relationship with him--for that and MANY other reasons)
But, as with many parts of the publishing process, whether your book will be available in these formats is almost entirely out of your control. You can certainly mention it to your agent but under ZERO circumstances will you make this a deal breaker.
And I should remind you that many people use electronic books, which can alter the font size, in lieu of buying or borrowing (from the library) large print books. Most publishers will produce an ebook simultaneously with print, so your desire to be accessible can be met that way.