No, Sandisk does not offer a player that accepts the SDXC format natively. And with the direction the company seems to be going, I doubt whether you will see one. The dedicated mp3 player market is dwindling as more and moe people are now using their phones or other multi-media devices to play their music. Personally, I believe we will never see another player like the Clip series or Fuze from Sandisk again. They have moved on.
Yes, a Rockboxed Clip+, Clip Zip or Fuze will take an SDXC card. But then so will using the orignal firmware too . . . if you re-format the card to FAT32 instead of the exFAT file format the card natively comes with. RB won’t recognize the SDXC format either, so if this is your only reason to try it, don’t bother. Rockbox has a lot of other features and options not present in the OF (original firmware) so it benefits a lot of people. But it is not for everyone.