See here:
http://www.iatatravelcentre.com/TR-T...ns-details.htm
NAV is an airport with Customs, so they can send the baggage through. Just ask nicely. IAH check-in is outsourced to (IIRD) Menzies but there are 2 or 3 TK folks around (normally at the C-class check-in desks). Ask them nicely and they waive the excess baggage fee on domestic segment - they are really nice...
PS. If you are eligible to use the lounge, it is United Club in Terminal E - Terminal D and E are interconnected so it is 10 minute walk from lounge to the gate (normally it is Gate 11/12)