Well, If you want a watercooled lan rig, you have to plan for it. Water is heavy, so using a resavoir in you system isnt advisable. Hose Clamps are. The fill bleed tube should be sealed off when being transported. Draining the system beforehand would be way too dificult and time consuming. So yes, you could carry a water cooled computer to a LAN. However special percautions should be made to prevent problems, such as the hose clamps and the cap on the fill line.
the most heavy single peice of my system would have to be the case cover ... its one of those 3 sided "armor plated steel" deals. well , not armor plated , but it does feel like it could stop a bullet i broke so many dremel disk bits that i just used a die grinder to make a window , and even after removing a 11x12 plate from its side it still weighs a ton ... so i added a set of office furnature wheels to the case and now i push it to the next lan party
yeah you can take your computer to lan parties without a problem, you just have to make sure its all inclosed and that nothing is going to leak all over the place while on a bumpy road
my computer is quite heavy so I made a handle out of a towel hanger I got at homedepot for like 5 bucks. this also allows me to carry the computer around verticaly without a problem. I would advise not to have a big res at all because water does get quite heavy and its harder to seal thin a T res.
I actually brought my Lian Li PC71 watercooled system to a LAN. It turned out alright but you have to be very careful when moving it(especially with plastic hose clamps) so it doesn't leak. A full tower case is also a bit awkward to carry. If you are going be building a Lan rig I would recommend a Mid-tower Antec/Chieftec case(an aluminum one). I'd think those would be ideal for a Lan rig with watercooling as they aren't too big.
A good thing to keep in mind is that loose components in your case are a no no at LANS. Make sure all your stuff is screwed into the case. PUMP and RAd should be bolted into the case to prevent them bouncing around your case when your carrying it.
My latest watercooled lan monster is a tiny Aluminum Coolermaster case. Has only 3 x 5.5" bays and barely fits the pumps and rad.
But It is much easier to carry around to the city for lans. And you get alotta OOooooo's and AHHHHhhhh's when you show up with a h20 system at a LAN. (chicks dig it too - LMAO)