Jack, I have been scanning 11.25" rolls for some time using a flatbed
A4 scanner and my custom stitching software.  Some time ago, I read
(in an AMICA bulletin?) a desperate call for people with wide scanners
to archive a corpus of deteriorating wide rolls, so I bought an A3
format flatbed scanner to prepare for this.  However, nothing ever
seemed to come of this, and I simply dropped the matter, putting the
scanner on a shelf for a later need.

As it is, I'm currently between houses and that scanner is deep in
storage, so I won't be able to help you right now.  But I do remember
that there are other scanning people using flatbeds, none of whom
I can identify for certain here, but who might chime in.

Marshall Jose