Well, guess what? (the tension mounts....everyone is wondering... :-)
Look at the pic:
That's the same adapter I used with my DX100. So, The AK120 DOES support native (exFAT) 128GB. I have 2 of those adapters. So, theoretically, I could put 2 x 128GB SDXC cards in for a total of 320GB storage!!
My next test is to stick about 120GB of music on that 128GB SDXC card, then rescan it in the AK120 and see how well it works. The picture above is just the empty card (no melting, no smoke, no Sansa Clip :-).
I don't plan to use the adapter(s). However, I'm within about 3GB of running out of space now with 2 x 64GB cards. So, as I get new music, either I down sample some of my hi-rez music to make more space (which kind of defeats the purpose of having a hi-rez player), or I remove some to make space for new, or I use the adapters. One of the reasons I sold my DX100 and bought the AK120 was to have something smaller, lighter, and without external connections. Decisions, decisions.
The bottom line, assuming it works when I test it with the card filled up, is that when 128GB microSDXC cards do come out, they should work in the player.