I'm itching to make an Irish stout, so I now own a nitro faucet and I added an external beer gas tank to my Sanyo 4912 kegerator a few minutes ago. The whole process took less than an hour.
I wanted to use an external tank so I can have a CO2 beer on tap as well as a nitro one at the same time. Setting up to use an external tank in addition to the internal CO2 tank is a snap and requires no modification of the fridge itself (excepting pulling out the rubber drain plug thing, which I did when I first modded the fridge).
Here's the 75/25 beer gas tank, a used repurposed 20 cu ft nitrogen tank I bought filled from Richmond Oxygen for $83+tax ($65 for the tank, $18 for the beer gas). They paint them black I suppose for easy recognition. The regulator will have a gauge cage on it as soon as I get it - the one I was sent was missing the bolts.
I used an inline QD on the hose so I can remove the excess hose and tank from the fridge if I need to move it. It was hard to push the hose through the drain hole, so that's also about as far as I wanted to yank it through the fridge.
The hose goes up through the drain hole, along with the temp controller probe wire and the blower cord that was already installed. The drain tray can be removed for installation - it just pulls off.
And finally, a view of where the hose comes out in the fridge (behind the CO2 tank, which is removed for this photo). I can keep it coiled on the shelf beside the internal CO2 tank until I need it.