It seems kind of odd that ssh-keyscan does not offer an equivalent to ssh-keygen's -r to easily generate SSHFP fingerprints for more than one host, without logging into each host. 
All the information needed is already fetched (i.e. the public keys) or known (i.e. the hostname), but as is, you'd have to generate the known_hosts output and then parse it again and hash it yourself or create temporary files for each line, as you can't pipe into ssh-keygen.

I realize that this mass-generation pretty much only occurs when you initially commit to deploying SSHFP, but all the code is already there…