Tab targeting doesn't really work against it there are multiple oponents in the arena - BUT;
Concecrate
Holynova
Deathknigth ground thingy
Arcane explosion
all will interrupt it and are use all too frequently - still this trick is pretty durn sweet.
Now while gnomes can leave their corpse there and res inside the box there is a way that should work for almost every race to do the very same thing.
(not this is untested as I actually only just though of it now )
8x
Pygmy Oil when drunk in rapid succession will turn you into a gnome!
so;
1) Gnomify your self
2) Log out at the chest position when nobody is arround
3) log in and loot
4) Profit ;-)
Now there is one thing that is of concern here the pygmy gnome has a rather tall mask that migth stick through the top of the box.
Luckily there is a remedy for that as well -
The bunny maker! [edit: if you foolishly threw away this awesome item it's possible that you can remove the mask by swapping / removing your regulare head gear - atleast that used to work for the deviate ninja]
This removes the mask and give some ears that are a lot less visible - and unless they fixed it removing the buff leaves you mask and earless
One question remains - is the pygme gnome's model so tall that your name will poke through the box?
oh and the trick is self obviously works best for shortnamed gnomes with short/no guild name as long names would stick through the sides of the chest depending on where the viewer is looking at your from.
Sadly I wont have time to test this untill sometime mid next week :| so if anyone tries do report you success / failure here.