Creating a group MMS application probably would not be a big deal, it's practically just the way messages are indexed together when shown in the UI and creating a list of messages-to-be-sent automatically when replying to the group.

Possibly something anybody interested could whip up in a few hours and polish up to publishable form in days.

It's just matter of nobody taking interest in it, maybe there really are no active SFOS developers in NA? As you know, people tend to make applications that they themselves need and as I previously stated I had not even heard of such a thing before reading this thread, GroupMMS is just not used in Europe...