That question is difficult to answer.

I tried on a mac with various SDHC readers. Sometimes it works, sometimes it doesn't. When it does not work, the mac complains that the file system is damaged but can read at least part of the files nevertheless.

The mac has a built-in SDHC reader which predates the SDXC specification, but the cards can be read just fine in that reader. The card can also be read in an even older reader from Transcend (manufacturer of the card).

So it seems that SDXC cards are electrically compatible, but that some new software or firmware would be needed to make the N800 able to read them.