Have a look at R V Rogers web site - I got a quince form them recently
and if I remember rightly they were advertising some on hawthorn and
some on quince rootstocks. I might be completely wrong though.

As far as I can see they only do Quince onto Seedling Pear or Quince A
rootstocks.

I am sure I have read it somewhere, but for the life of me I cannot
find it.

Now seedling pear must be a 'Pyrus communis' and Quince "Cydonia
Oblonga" was reclassified a few years back and used to be called
"Pyrus Cydonia" . So they are both in the same family related to
Rosaceae.

So I assume they are compatible, but I still seem to think there was a
problem.