plus this code needs to have null bytes, plus it needs to avoid buffering which still hasn't really got a solution yet
Null bytes? You mean packets containing 0x00, or you mean packets of length zero? I didn't realize the Ethernet library disallowed the former, and didn't realize TCP allowed the latter. 
As for buffering, sounds like you need a Nagle's Algorithm switch. If enabled, allow output buffering but a timer will send the current buffer if you don't explicitly flush() to send what you've built up.