No but I've thought about it. I bottle from kegs a lot and a sight glass would make my life a lot easier so I knew how many bottles to sanitize.
Several things on this, I use nylon bulkhead fittings from mcmaster http://www.mcmaster.com/#8671t22/=pakfl8 (nylon is still a food grade material) and a silicone gasket on each side of the keg http://www.mcmaster.com/#4520k41/=pakhtk. then you simply buy pushlok style nylon fittings, http://www.mcmaster.com/#5228k21/=pakj1r these fittings hold hose up to pressures of 150psi, then you can use normal beer tubing. Tools required 1/2" drill bit 2 crescent wrenches ptfe tape heat gun or hair dryer Directions apply 2 layers of the ptfe tape to the barbed fittings tighten until they feel as if they have bottomed out in the bulkhead fitting Drill x2 1/2 inch holes in the side of your keg at the top and bottom. Install the bulkhead fittings and place the silicone gaskets between the nuts and the wall of the keg on both sides tighten until the gaskets flatten completely do not over tighten, they are nylon fittings you can strip the threads off, let the silicone do all of the sealing work measure the tubing so it is approximately 2 inches longer than the distance between the fittings heat up your tubing with a heat gun(this step is not required but makes life a lot easier) slide the tubing onto both barbs until the edge of the tubing reaches the bottom of the fitting DONE Cleaning, make sure you clean the bulkhead fittings each time to prevents sediment from building up in the bottom fitting. If you pressurize your keg and it leaks around either bulkhead fitting, then the silicone gaskets are making a complete seal, you can always add http://www.mcmaster.com/#7545a471/=pakt27 which is a food grade silicone sealant.
I have counter's attached to my keg taps, so every beer i pour I just push the button. When it gets to 52, I know the keg is about gone. Plus it gives me a great reference point as I am getting through the keg....it's this model http://www.ebay.com/itm/like/200783837066?lpid=82
Negative on the photo, I don't use it anymore, When I first started I did, but now I use the old school bathroom scales. I place the empty keg on the scale, zero it out, then fill with beer. Once it get's close to Zero I know I'm getting close to running out. I do however like the counter idea. I've actually got a few of the old click styles in the classroom, I might borrow one. The scale can get hard to read under 5lbs. Sunbeam is a scale that i use, but any dial bathroom scale works, this is just the one they had at Wal-Mart, the clickers are cheaper though.
Just leave your keg out in the middle of the room for about 20 minutes and you'll see a nice and clear line of water beads that stops at the level of the chilled beer.
There are several manufacturers who make temperature-sensing stickers that indicate the level of beer in the keg. Here's one: http://www.ebay.com/itm/ws/eBayISAP...9683&item=290802299683&lgeo=0&vectorid=229466
