One potential problem with having the radiator inside as others mentioned will be dehumidification of air in the terrarium. But if you are running the humidifier constantly it should not be a problem. Humidity, could damage the fans if they are in a very wet air. But if you leave the radiator somewhere closer to the top I'd think it would solve the problem.
I am sure others with more experience and knowledge can point out other issues with this approach. But the potential benefit is that your cooling will be faster and more efficient as you will not be cooling hot air from outside and you could reach lower temperatures.
Last year when I did my AC mod to my tank (see link in my sig) I placed the Rad right in the tank. It really lowered the RH a lot, about 20-30%. The Humidifier would try to compensate but wasn't strong enough. the plants were fine though for the most part.
With the new setup, using a smaller rad, you shouldnt have much of a drop in RH. Plus is looks like you have a pretty hefty humidifier so it should have a problem compensating for the drop in RH. Moreover, you wont lose as much RH because the temperature of the radiator isn't all that cold. On the AC I used, the rad would get to like -30c in a few minutes, and would frost up really quickly so I put a plastic box over it with some holes in either side and it was much better. So what I'm saying here is, I'd stick the rad in the tank, as gil_za pointed out, the temps will drop quicker and it's also a lot more efficient.
And what is interesting is that I've done a quick search on google and haven't yet found similar setups with water coolers. So robthered's idea might be unique He should post it on "instructables" or something.
I'll definitly post a how-to up there once I get it all finalized , thanks for the idea.