I've been kicking this idea around in my head, and maybe this will work for you:
I've been looking at my blichmann dial thermometer. The mechanism that is uses to seal is just a 1/2" NPT threading around the probe. You could take a stainless 1/2 bolt and cut off the threaded portion. Then drill out a hole down the center such that the probe can be threaded through. Finally, just fill any gaps with silver solder.
Then you could install it just like a weldless dial thermometer using a SS nut.
I'm going to try doing this with a cheap walmart thermometer so I can have a digital reading on my keggle mash tun. (unless someone chimes in to tell me this is a bad idea.)