iRule to create TPDU header

You are heading in the right direction. A few small things right off the bat:

1. Generally you don't need to set IP::client_addr, TCP::client_port and TCP::payload to a variable. For Tcl, it is faster not to do so (particularly for the payload);
2. You don't need to unset variables. Variables are scoped to the connection (unless they are declared global -- but don't do that -- or in the static:: namespace) so they will be freed when the connection closes.

With those quibbles out of the way: what is the server expecting? Does it want 32-bits (the IP) followed by 16 bits (the port) followed by the original data, or does it want the IP address as text, the literal colon, then the port, as text? Also, should this data be inserted at the start of the TCP stream (that's easy) or with each TCP segment (that's difficult, maybe impossible)?