Can I share an exFAT partition between OS X and Boot Camp?

Yes to both. exFAT can be read and written to by both Mavericks (as well as Mac OS X 10.6.5 or later) and Windows 8 (as well as Windows XP SP2 or later). Windows can recognize all partitions created by Disk Utility, even those with filesystems Windows can't read.

Edit: Niel's answer is better. I forgot to warn you that the partition would have to be on a hard disk different from the one you're running Boot Camp on.

1. Yes, but not on the internal drive; creating a third partition on it, if allowed, may disable Boot Camp.

2. See #1. The best choice is to use an external drive and format it from Windows.

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I've got 2 storage partitions, bootcamp and OSX running on the same machine, internally i.e. all on the same drive. A total of 5 partitions, if you include the 200MB EFI partition.

Guide:

Attempting to edit partitions in OSX will break Bootcamp. Make sure to edit partitions in Windows. I used Partition Wizard (PW), I recommend you use it, Window's default partition editor is pretty nasty.

Before you start, it's best to have the empty space you want to use for your new partition AFTER the Bootcamp partition. So create your NTFS Win8 partition with lots of space so you can shrink it later. After you've got Bootcamp set up with the free space you're going to use for your FAT partition as part of your NTFS Win8, you're ready to go.

Shrink your Bootcamp partition with PW and then create however many partitions you like in the filesystem you need. Pretty straightforward

I'm running Win8.1 and OSX with my partitions in this order: OSX, Win8.1 (Bootcamp), 10GB Storage Partition (NTFS), 90GB Storage Partition (FAT32). All my partitions are accessible by both OSes.

exFAT is basically the same as FAT32 except you can have file sizes greater than 4GB.

System: MBP Retina 13" (Late 2013) 8/256GB

I just tried this but I kept getting an error "No free MBR slot."