The only thing I can share here is that I honestly believe there are no intelligent solutions to stupid requests.
Apologies for the rough language...
Apologies accepted, Olaf. However, I hope I get some helpful input that would help me solve this issue. If it can’t be done, I need to explain why...
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.
I actually created the "Document" tag PDF, which is the one where I get the “Warning, empty page” message and is the one you are suggesting first, but I haven’t tried tagging all content across all pages. I will try that and see how it works. Depending on the outcome, (and for the sake of practicality), I could then make an informed decision to whether keep trying something else or not. Time is precious for this project.
I agree, client’s request is rather unusual, but I welcome the fact that learning what the technical limitations are to fulfill this request makes this issue rather unique.
Your reply really helps, Olaf.!