You are speculating that changing the advertised MSS would affect the sender buffering, but that AS perl doesn't support it. Have you actually tried this from some other OS that does support it?

You've said that you cannot change the sender, but then you've just given (fair) consideration to the idea of using UDP. Wouldn't that have required changes to the sender anyway?

The very low transmition speed suggests that the sender is either very far away, or using a very low power transmitter? A little more information about the hardware/software setup might bring further possibilities to light.

I am always amazed by the breadth of the backgrounds of the guys that frequent this place. It's just possible that with a more high level description of the problem, rather than questions of how to implement one possible solution, you would engage the interest of someone with experience in the field.