If sms:// launches Messages (and it does), then you don't need an mms:// handler, because sms:// launches Messages but doesn't determine the message type. Simply tap the camera icon and choose the photo to send once Messages launches.
Protocol handlers like tel:// and sms:// are just glorified urls, like http://www.askdifferent.com/, and they were not designed with multiple resources in mind. Resources being domains, files, etc. Note that you cannot request two files from within a single URL.
Example: You cannot do something like: http://www.askdifferent.com/images/uploadedscreenshot1.png+uploadedscreenshot2.png
I've attempted various delimiters for multiple recipients, such as %20, ,, |, +, and &, and none of them working. I would not be surprised if there is no way to specify multiple recipients via the sms:// protocol URI.
