On an Android phone, my assumption is that since the phone itself handles the SDXC card then this should work with RB on that phone (but that's very much an assumption).
Yes, as in any hosted environment Rockbox does not do the "bare" filesystem operations. Right now there's even this situation: on Android the SD card is (usually) FAT, but everything else (usually) uses a different file system. Rockbox does not know about those since those are handled by the OS (i.e. Android)
If/when SDXC becomes more prevalent on the devices that RB runs on then I'd be frankly shocked if RB did not support it.
You might want start being shocked then.
For SDXC cards, there are two types available. Version 3.0 is somewhat
compatible to SDHC: it can physically be used with SDHC hosts. However, the SDXC standard requires those cards to use the exFAT filesystem which Rockbox does not support, and is unlikely to support since it's proprietary and requires a fee to be used. One can reformat such a card to FAT32. In that case it should work with Rockbox (except that Rockbox cannot use the new features of SDXC, as is true with any SDXC card reader). However, other devices (like digital cameras) might not be able to handle an SDXC card that uses a different filesystem than exFAT.
The newer type of SDXC (haven't found the spec version right now, I guess it's 3.1) is not
compatible to SDHC so it is not
possible to support such cards unless the player has an SDXC host interface. Given that evolution of music players goes rather into the everything-device direction (like Android and iOS) I doubt a player with SDXC host will ever be available. But with the (likely) evolution of Rockbox to running as an app this won't be much of concern anymore.
If a player comes out that has an SDXC host interface it might be supported, but supporting exFAT is highly unlikely.
(and as a side note, this SDXC thing is -- from a technical point of view -- unnecessary when it comes to storage sizes. SDHC changed the addressing compared to SD, and the protocol does not limit the maximum size to 32GiB. However, the SDHC spec limits the maximum size to 32GiB. In theory it's perfectly possible to build a 128GiB SDHC card, but that's out of spec so this will never happen. Plus, vendors need a reason for us to buy new card readers and other devices ...)