So.. why? ExFAT really isn't designed for normal disks and is optimized for flash drives (i.e. SDHC). I assume this is for drive sharing between Windows and OSX but honestly NTFS is way more robust than ExFAT.
ExFAT was already supported in ML so I have no reason to believe it won't be supported in Mavericks. Don't have an external drive handy to see what disk utility can do.
http://en.wikipedia.org/wiki/ExFAT
Also, according to this post:
http://prateekvjoshi.wordpress.com/2013/04/21/ntfs-write-support-on-os-x-mountain-lion/
ML had "hidden" support for writing to NTFS anyways. I have not tried this as I don't have an external drive that I am about to format NTFS. I may however try to find a drive at work and see if 10.9 has better support for NTFS. I'm a little curious now.