It is probably on of the "most requested" features but I still haven't figure out a nice way to implement it. I am hoping that the developers of FastSMS, VertSMS or any other alternative-SMS applications would implement the feature and I would just change my contacts to send a DBus signal to them with the list of phone numbers.

Otherwise, I will have to create a "front-end" to allow users to enter the message when I think we have already too many good options out there.

I would suggest you (and everybody else interested on this feature) to contact your favorite SMS application's developer and suggest my approach. I will gladly work with any of them to make it happen.