UPDATE: I now have 128GB in my Kobo Aura H2O: 64GB internal + 64GB external. I was quite surprised to discover that the external slot actually supports microSDXC cards.
Now, the next step is to try 128GB + 128GB = 256GB total. I will try that as soon as my 128GB cards arrive (hopefully tomorrow). Btw I got very cheap 128GB microSDXC (but from an official SanDisk retailer, so a fake is unlikely but I'll check it of course) here:
http://www.ebay.co.uk/itm/371093783490