Is there a URL scheme for sending an SMS to multi-recipients?
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 &amp;, and none of them working. I would not be surprised if there is no way to specify multiple recipients via the sms:// protocol URI.