Turns out I lied. I tried to image the Win partition with NRH just now and had nothing but failure. I found a post on the forum that said to try it from the CLI, like this:
1. Get the list of disks, note the one with Windows
diskutil list
2. Check to see how much space we could save if we resized it
/usr/local/sbin/ntfsresize --info -f /dev/disk2s3
3. Clone to Windows partition to a disk image
/usr/local/sbin/ntfsclone --save-image -o windows.img /dev/disk2s3

First run through was a test. Results:
Rebuild:sbin admin$ sudo /usr/local/sbin/ntfsresize --info -f /dev/disk0s3
ntfsresize v1.13.0 (libntfs 9:0:0)
Device name : /dev/disk0s3
NTFS volume version: 3.1
Cluster size : 4096 bytes
Current volume size: 31868277248 bytes (31869 MB)
Current device size: 31868280832 bytes (31869 MB)
Checking filesystem consistency ...
100.00 percent completed
Accounting clusters ...
Space in use : 14566 MB (45.7%)
Collecting resizing constraints ...
You might resize at 14565183488 bytes or 14566 MB (freeing 17303 MB).
Please make a test run using both the -n and -s options before real resizing!

So I did a test run:
Rebuild:sbin admin$ sudo /usr/local/sbin/ntfsresize -s 14565183488 -n /dev/disk0s3
ntfsresize v1.13.0 (libntfs 9:0:0)
Device name : /dev/disk0s3
NTFS volume version: 3.1
Cluster size : 4096 bytes
Current volume size: 31868277248 bytes (31869 MB)
Current device size: 31868280832 bytes (31869 MB)
New volume size : 14565179904 bytes (14566 MB)
Checking filesystem consistency ...
100.00 percent completed
Accounting clusters ...
Space in use : 14566 MB (45.7%)
Collecting resizing constraints ...
Needed relocations : 902174 (3696 MB)
Schedule chkdsk for NTFS consistency check at Windows boot time ...
Resetting $LogFile ... (this might take a while)
Relocating needed data ...
ERROR: Extended record needed (5232 > 1024), not yet supported!
Please try to free less space.

I need to come back to this. In the meantime it's back to Winclone.