A further update. Despite the text looking good/okay in BookWright which is used by Blurb to print their small-volume book runs, the printed article looked really bad. The proof PDF I checked meant I opened a support ticket with Blurb but they said the PDF proof process was low-res and might not look that good.
I went ahead and did a sample book (albeit in my desired run of 1 book!)
The font shown in the book has jagged edges, the characters don't line up horizontally but go up and down with each character, commas appear as apostrophes... so I have reopened by Blurb ticket.
None of this is the fault of the OpenDyslexic font (which Blurb had said could be used if it was installed on my Mac as I suppose they embed it in the document), but what it does mean is that you will need to be careful if you are planning to do the same thing I was doing. I may have to revert to a built-in font which has dyslexic readability but at least I have the time to do this before I need the book.
(Blurb delivery is at least quick and the article is nice, albeit the text is not)