Hehe yea really do think about your loop more than once or twice is my advice. Even do some windows paint or a 3D program work does help.
I use pen and paper and draw several loop`s just so see if the loop looks dumb or not + drain, flow etc etc.
When it comes to doing sli in vertical... To be honest I dont know about 1 guy who has done it. But sure it can be done but you would need a sheet of aluminium 2 - 3 mm thick since you dont want it to sag from the weight of the 2 cards.
Do a 90 degree bend on it and fasten it ofc to your case with some screws or even weld it in place.
Then you need 2 of this one: http://www.thermaltake.com/Chassis/Accessories_/_/C_00002784/PCI_e_X16_Riser_Cable/design.htm
or what ever brand or looks you can find / want
Cut out enough room so you can thread it through the hole and fasten it somehow ( glue, screws... Something
Watch the spacing so you can get on an sli bridge ofc
ofc there is more steps but if you want to do the work sure go ahead
If it is worth it on the other hand is something else. Since you would in a way "block" one of the card and you would hardly see it. You will see 2 cards better in sli mounted horizontal than vertical. Vertical is nice to show the waterblock and flow of water. Dont see the need to do 2 unless you are hell bent for sli and the job it includes.
But doing it I am sure you would have a rig not many else has since the work you need to do. Unless there is some kind of pre fab bracket out there I dont know about that support sli in vertical