Is there a way to force a message to be sent as an MMS?
As per Broam's suggestion above, I'm posting my comment as an answer. Hopefully some people finds it useful.
If you add a Subject to the message you're making, it gets converted to MMS format. Even if the Subject is empty, at least, that's what I know
Force the MMS
I've saw this comment you provided on your question:
If this is the case, you can have a
As far as I know, you can't.
An MMS message is a plain old text message with contents attached.
If you attach a media file, it will be send as a MMS message. Otherwise it will follow as an SMS.