Accessible PDF - a challenging issue?

From a purely syntactical point of view, you should be able to apply the Alt attribute to one tag (try the "Document" tag, or create a "Part" or "Section" tag around all the tags in the document) enclosing all the content (across all pages). I have no clue whether Read Aloud will handle it as expected (and it operates differently in newer version than it did in older version; the newer versions strictly follow the tagging structure, whereas older version followed sequence of content as encoded in the page description [what you would see in the Reading Order panel]).

For the sake of clarity: your customer's request has nothing to do with accessibility but rather with achieving a very specific effect. Read Aloud is not a mainstream assistive technology (I guess that even Adobe would not claim anything in this direction), most people relying on text to speech based assistive technology will probably rather use JAWS, NVDA, WindowEyes, ZoomText, VIP Reader, and so on. It would be more important to ensure that those tools provide reasonable access to content rather than to focus on Read Aloud alone. In addition there are more people having a disability not related to vision than there are vision impaired people. Reducing access to a document's content to one Alt attribute could hit these people in unfortunate ways.

Olaf