I dont understand your question. Exactly what library are you using?
MSS and MTU are set in the OS and usually no one tampers with them. If you give a bit more background I might be able to help.
From what I understand the send() function or the regular python standard library http://docs.python.org/library/socket.html may not be able to set these values.
For one, you would need certain privileges to change these values, I would look at OS related package for that which can make calls to OS before you initiate the send command.
thats all I know