Long post to answer several of the items in the thread
Copper should work fine for the jockey box. Contact time is minimal. I would suggest running VERY hot water through the coil before and after as a cleaning and surface stripping measure. May not initially make a lot of sense to strip off the oxidation of the copper, but think through it. If the surface is actively corroding yes, you may pick up some copper ions in solution as you dispense your beer (chances are it's in you home water supply too!), but that's far less noticeable than if the line has corroded (tarnished - as copper is wont to do) and you scrub off chunks of tarnish and copper oxide (which is not as soluble in water/beer). Idea being, reduce the effects of the copper flavor potential rather than trying to prevent it from corroding (tarnishing).
On the poly... you'd have to get some SERIOUS heat on that corny to keep that thing coiled after you take the heat off. The coils of poly (actually polyvinyl chloride - yes, it's still PVC) you buy at the hardware store are coiled as they're being dried/cooled. They may be later heat treated, but we're talking a heat treat of >100C under very special conditions (sometimes vacuum). I wouldn't try to replicate that process.
What I would suggest (that's easier anyway) is buy a couple good lengh coils at the hardware store and leave them coiled. Or recoil them and tie them up with twistie-ties or whatever. O.k. so you've reduced the surface area for heat transfer on the tubing, o.k. so PVC is not a great conductor of heat anyway (remember you make it colder by taking away heat - not adding cold - see the 3rd law of thermodynamics for further elaboration), but PVC is cheap. Really cheap. Lots cheaper than copper. Easier to work with. Let's say - for argument - that you were gonna use 6 feet of copper coil (probably on the low side, I'd use closer to 10 - the more tubing you use, the closer to the temperature of your cooling medium - ice - you'll get). Copper conducts heat better than PVC, so if we're going to use PVC let's doube the estimate - 12 feet. O.k. so let's leave it coiled or tied up (if you're really ambitious, re-coil it in a single layer instead of dual layers - you'll see what I mean when you buy it at the hardware store), we've reduced the surface are of the tubing by ~1/4 given all the touching it does in the coil. Let's add another 50% to our estimate - 18 feet.
Last time I checked at Lowe's Hardware 5/8in PVC tubing (1/2in ID) was 20ft for ~$3.75, same length in copper ~$8-10. Plus you don't have to worry about off flavors and the fittings are cheaper. In fact if you buy a cheap styrofoam cooler at Wal-mart you may have invented the world's first disposable jockey box. It's a good tip for a party if you've got a warm keg, it takes about 30 minutes to put together and you throw it away when you're done.
Anyway, just my $0.02
Hope this helps.