If everything that is fabricated/assembled weighs less than the allowance, you're free to do it. You can get creative here by removing any parts that are (unmodified) COTS and putting them back on at the event. I'd be careful about getting into the situation where you spend all day Thursday assembling the robot, though.
I remember getting some nasty looks in 2009 when the entire upper half of our robot was within the allowance that year, and we were waiting to walk in with what looked like an entire robot
(Usual caveats apply, re-check rulebook and Q&A for any items you intend to withhold)