In place of mPDF (T9582), I'm moving right ahead to the only PdfBook alternative left, DocBookExport. As stated in the linked filing, DocBookExport was once an experimental part of Referata, and furthermore boasts multi-page support unlike mPDF.
This requires the "Figures" extension, however--but that's in beta at this writing, and as @Robkelk expressed his concern at the Noticeboard, "who knows when a change to Figures might make DocBookExport unusable?"
As before, the likes of DBE would sure come in handy for my future books. Either that, or we'll have to give up until further notice and resort to an external/third-party tool. To Reception and/or UO: How promising are the chances this time?