I have, and it does work, but doesn't cool to the extent I had hoped. I think I simply had too much 'metal' exposed to ambient temperatures to make it work efficiently.
Basically, I had it set up on a 6.5 gallon Clawhammer keg with a removable 4" TC lid that has Gas In/Gas Out Cornelius posts, a ring-pull PRV, and a 1.5" TC port. I attached a 1.5" TC cross (4 x 1.5"TC ports) to the keg lid. The CoolStix passed through the vertical (top) port into the kegmenter. I attached a Spike spunding valve with a 1.5" TC 90^ elbow fitting to one side of the cross, and a 1.5" TC PRV to the other side. Then I connected the CoolStix to my glycol chiller. I've got three outputs to chiller coils, but at the time I was just using one (occasionally two) cooling output channels simultaneously. When I first used the setup, I had my unitank maintaining lager temps on an active fermentation, and one connected to the CoolStix.
Ambient temperature in my brew area runs about 55-60F in Winter, and 65-80F the rest of the year. The lager in the Unitank was fermenting around 48F, so some cooling load, but not a lot (it was a cool Spring outside). I had an ale going in the Clawhammer and the CoolStix was doing O.K. maintaining low 60s, but when I attempted to cold crash, it struggled mightily to get below 48F.
It wasn't that the glycol chiller wasn't up to the task. I've had two circuits chillin' and one circuit 'cold crashing' at the same time down to mid 30s, so I know it has the muscle to get 'er done. The CoolStix works well in the limited capacity range between say 50-65F with ambient temps in the upper 80s outside. You'll have no problem with fermenting ales and lagers, but don't expect being able to do a hard cold crash or lager in the upper 30s. I'm sure my setup with about 8-10" of extra stainless steel hangin' like a Christmas tree didn't help the efficiency by sapping cooling power, although I did use a neoprene sleeve on the keg.
In short, the system you described does work, but it doesn't work miracles, I was looking to add capacity and capability without spending money on another pressure capable unitank. If you want it to bend the temperature curve, it will. But you'll have to insulate your keg while also limiting the amount of exposed SS hardware on top, and have a reasonable capacity cooling system (glycol) to change the delta on ambient temperature to where you want the fermenter/keg to be.
