if it is the "new" BFC script that supports format hints, then juan explicitly took out the multi-fits support. i cant remember now if the new types used to represent the image (the ones that support format hints) can't support multi-fits, or if it was just 'laziness'. i had hacked on this script a little bit and released it to juan before even trying to add multi-fits support and he didn't seem interested in fixing that up.
if it's the old script, then it did have support for multi-fits.
not sure about the exception you are getting.