Just put it in the freezer, laptop on the top of the freezer, very thin usb cable. Cooling done.
Unfortunately heat transfer rate and absolute temperature differential are not the same things and do not have the same impact on cooling performance. It could be in the freezer at -18 deg C but as soon as you apply power to it, that power has to go somewhere as heat through all the materials that surround the ASIC die. It's entirely likely that the die is still sitting at 50+ deg C.
I'd be willing to bet that you could remove more heat effectively with watercooling at ambient temps than running a half-assed sub-ambient cooled system in a freezer at -18 deg C, even if it was actively cooled inside the freezer (with fans).
At sub-ambient temps condensation is a real threat to electronics and I'd be willing to bet if you just put an erupter on a USB extender cable* and slide it into your freezer it will be destroyed within seconds. If you went to the effort of sealing the overclocked erupter in an almost perfectly dry isolation vessel, with heatsinks on both the top of the chip and on the PCB underneath, with a fan inside it to circulate the air, you might have good enough heat transfer to cool the chip better than with just a crude watercooling setup. But generally in that setup you are going to be limited by two things: The thermal conductivity of air is very poor, and the heat must still pass through the isolation vessel itself.
The difference with water cooling is that you have a very high heat capacity medium (water) flowing past a metal surfaces (waterblock) at a high flow rate. The metal itself being copper (or brass in my idea above) or aluminium in most cases also has exceptional heat conductivity that is orders of magnitude better than air.
On the other hand, if you had large enough heatsinks removing the heat into the freezer and the metal directly conducted heat from the ASIC chip top/bottom then you would get superior cooling. But I think it would be a huge pain in the ass. It's times like this where die-mounted temperature monitoring like in CPU and GPU would be great, but considering how much the AM ASIC chips cost it's to be expected that they don't have this feature.
If the erupters had built in temperature monitoring on the ASIC die I would have already tried putting on in the freezer just to see what kind of temps can be obtained. However because monitoring the external chip temp is almost meaningless there is little incentive to try. Don't even get me started on actually trying to get high accuracy reproducible measurements of surface temps with thermocouples or IR pyrometry...
I suppose some good news is that as far as I can see none of the components on the erupters are highly susceptible to damage from sub-ambient temperatures. As many people have found out trying to do 'extreme overclocks' on graphics cards, the electrolytic caps don't enjoy being frozen solid. It changes their electrical properties too much. The spec sheets of the major components do state minimum operating temps and many of them are -25 deg C although I wonder if the manufactures actually do failure temperature tests to determine that. I could understand them testing maximum temperature ratings of course, but minimum temp is a bit unusual in my mind and probably not worth their time.
If you wanted to submerge the unit in oil for your testing it will probably be alright as the only component on the circuit that could conceivably be affected is the choke, because the epoxy could be broken down and the magnet wire insulation degraded over time. Which can and does lead to some failures in large oil cooled power transformers.
*FYI, I have already tested running erupters off USB extender cables. I am currently running two erupters each on a 2 meter USB 3.0 rated cable, on a netbook. Interestingly I measure no voltage drop over the 2 meters and the netbook happily outputs 5.06 volts both at its hub sockets and also at the end of the cables while under load. USB 2.0 cables are usually rated to much lower current carrying limits so those might not be able to handle it. Then again 2.5 watts on a unmodified erupter is pretty small amounts of power to begin with.