Hi MMD,  About twenty years ago I did a fair amount of research
into this topic.  I don't remember the details but I tried three
different brands of spray starch.

There is a fair amount of research on the disintegration of paper.
At the time it seemed that the paper fibers might be rejuvenated by
putting starch back into the paper.  Suffice to say it did not work;
too labor intensive and time consuming.  Only short sections of paper
could be done at a time and then it had to dry before you could move
onto the next section.  Wet paper falls apart and dries very slowly!

Next I tried a couple brands of clear acrylic spray.  This left the
paper so stiff if could not be rolled onto the cardboard core.

A German company has developed solutions and machinery for libraries
for the restoration of books, but this process is expensive and not
really adaptable to piano rolls.

This is a topic that needs further study.  But you now know what
does not work.

Tom Hutchinson