[plt-scheme] bitmaps as bytes or ports

Using load-file and save-file it is possible to create and save bitmap%s 
  from file.  Is it possible to do the same with a port or byte string?

I am trying to write a function that takes an image-snip% and base 64 
encodes the contents of its bitmap in a given format.

(base64-encode (send (send image get-bitmap) save-bytes 'jpeg))

But I don't see how to do save-bytes.

David