Very cool. I've had some ideas like this, but have not tested them out yet. I bought some parts, and will do a trial 'soon', but I thought I'd get these ideas out there, you can probably run with it.
My general idea #1 is similar to yours, but INTERNAL to the fermentor. Pump chilling fluid into a thermowell inserted into the fermentor. Put your temperature probe on the outside of the carboy, well insulated from ambient (tape it and tape foam over it).
For a carboy, use a carboy cap and stick a SS thermowell device (buy, or make from SS or a chrome water supply tube - plug one end) in through the large hole, use a hose on the smaller one (or cut it open) to connect your airlock. Run a 1/4" OD vinyl hose from the pump down to the bottom of the thermowell. Now (and a drawing/pictures would help here, I'll try to post one later) you just need to rig up a PVC Tee and caps, and run the vinyl hose through one end, and the chill fluid would 'return' through the open end of the thermowell - a hose on the Tee would return this to your cooler. So it would be a co-axial setup, just break out the two streams for in/out, if that makes sense. This is all low pressure, so some silicon caulk should be good enough to seal it all up.
True, this would not be much surface area, but you have chilled metal in direct contact with the beer
. I bet that is better than the larger surface area going through glass. And you don't lose any cold to ambient, it almost all goes right into the beer. It also makes it really easy to insulate. I'd wrap that carboy with as much scrap bubble wrap as I could scrounge, and wrap the hoses. It should also reduce any condensation problems, the coldest parts are inside the fermentor, not exposed to outside air/humidity.
With a bucket fermentor, you have added options. You can get annealed SS tubing that is easily bent into a loop, and drill two added holes for grommets and just run the loop inside the bucket. I got some SS tubing on amazon from a company called 'small parts'.
My idea #2, would be to try some tire inner-tubes around those carboys. Add a tire stem on the opposite side, and pump the chill water through the tube. Certainly not as good as metal for transfer, but it might be good enough, and certainly easier.
BTW, your pump looks pretty small - and that is a GOOD thing. Most people want to put a big pump in there, but the pump creates heat and is counter-productive. The carboy can only transfer the heat at a fairly low rate, pumping the chill water faster than it can absorb heat really does almost nothing to speed the process, and the pump just melts the ice. Do you have a link for that pump?
Here is the pump I bought to play with:
It doesn't detail this out there, but it is a 12V DC pump, .8A xfmr, so it could be hooked directly to a cheap home thermostat, avoiding the cost of the controller (though you probably need to extend the thermistor to create a 'probe'), and also all low voltage around that water.
Hope these ideas help, I will try to get pics up later, but I hope the description was enough to get the ideas flowing.