I was able to create a working dual boot card per the instructions. However, I could not make a working card that made use of all 16 GB of my card, rather than just the <4GB size of the image file. The card still worked when I used fdisk on my Mac to increase the size of the last, 'sdcard', partition to fill up the available space. However, that did not increase the capacity of the FAT volume in that partition, leaving the extra space still unavailable.
After saving the files from that volume, I then used the Mac's Disk Utility to erase the partition and re-create it as a FAT partition using all the available space. Once I did that, unfortunately, the Nook would then not recognize anything on the SD card and would only boot into the Nook's own ROM.
As an alternative, I tried leaving the existing partitions alone and editing the MBR with fdisk to create a new partition using the available space, which I then formatted as a FAT partition with Disk Utility. Again, the Nook would no longer recognize the SD card and would only boot into its stock ROM.
I actually tried the above with two 16GB SD cards, a SanDisk Class 4 and a Samsung Class 10, and with two Nooks. Moreover, I tried a number of manipulations of the MBR, mostly to make it identical to the original one except for necessary size changes. Nothing worked to get a card that would boot and use the full 16 GB. I'll admit I haven't tried everything, such as trying to resize to something less than the full 16GB, but I've put so much time into this already that I don't want to do any more until I get some feedback.