Yes, this would be possible. However, I haven't done it personally and can see it being a little bit involved. From doing a search, it doesn't look like anyone has approached this either.
Creating the image is the easy part. But to replace the stock fuse setup / supplement it gets tricky. You'd need to extract the ramdisk from your boot and make some changes there at the least. Further, you'd probably have to decompile your framework to modify res/xml/storage_list.xml.
Overall, I think it's doable but would require some research comparing with sources for other devices (with SD cards). I guess it depends on your level of aptitude / desire. Might be better off attaching a usb stick to your keychain.
To get the ball rolling, here's a way you could create a vfat image:
dd if=/dev/zero of=/data/sdcard.img bs=1M count=1024
That would write 1MB of 0s 1024 times (1GB of 0s) and then format it as FAT.
Perhaps you could research having that "partition" be an add-on, supporting UMS, while still maintaining the default fuse setup.