Helene Bertrand, Angers, France, is the ICRA for Hydrangeas. Has a list of recognised or accepted cultivars ever been published? If so, how is it accessible? Has a list been published in English?
I do not think I have ever seen one but you would expect them to publish one with "some regularity" (maybe every so many years if not annually). Likely printed on paper though. Have you tried contacting them? The International Cultivar Registration Authority (ICRA) at the National Horticultural Institute, Angers can be contacted: Institut National d'Horticulture, 2 rue Le No^tre, 49 045 Angers Cedex 01 France
Their fax number is: (+33) 2 4173 1557
Helene Bertrand's email is firstname.lastname@example.org