[UnraisedArc]

Here is a thought that solves the problem a different way.

In the case that it is too difficult or complex to implement with calibre, what if there were a standalone piece of software written that processes files (.pdf, .lit, .mobi, etc.) by the method you described BEFORE adding it to calibre:

"load the text of the first 10 or so pages of a PDF and search it for ISBNs of this type by means of a user-configurable regex. If several matches are found (e.g.: "ISBN-10", "ISBN-13" and "eISBN-10"), the user might be given the opportunity to select one from a dialog."

If this software does find an isbn number, it could rename the file as the ISBN number. Then, when importing the files into calibre, you could simply use the already-existing feature that takes the filename and saves it as tags.

For example, pretend this imaginary software finds that Alice_in_Wonderland.pdf has ISBN 0123456789 and renames the file to 0123456789.pdf. In calibre you could then deselect the "Read metadata from files" option and change the regex to put the filename in the ISBN tag. Then, after you have added all the files to calibre, you could simply bulk download metadata, and since the ISBN number would be saved as a tag, it should come back with good results.

Obviously I like your method better, but if it can't work, maybe this could.

Thanks again.