Probably for EPA reasons, Kawasaki and Keihin conspired to produce and deliver Concours carbs sans overflow tubes. What this means is, if the float valve sticks open, instead of leaking gas all over the ground, gas trickles into the cylinder. Then when you try to start the bike, it comes to a sudden stop. If you're lucky, you just pull the spark plug and trickle the same gas onto the same ground. If you're not so lucky, it bends the connecting rod, and you take the engine apart and trickle the same damn gas on the same damn ground.
So first I grabbed a RHS float bowl, $5.00 US NOS from eBay (Sept. '06), so I could have a good look without taking the bike out of service. The Keihin CVK 32 carb float bowls have what looks like an unused boss exactly where a overflow tube is just begging to go. Moreover, if drilled through into the drain screw passage, this overflow runs out the existing bottom nipple regardless of whether the drain screw is closed.
There was a discussion on the COG Forum called 'Absolute hydrolock protection' in October '06, which I saved in text format.
Then (Nov. '07, some projects require extended analysis) bought one foot of 1/8" O.D. copper tube, $1.53 CDN all up, at a local hobby shop, it's used to make slot cars or RC helicopter frames or something. Brass, aluminum, steel, stainless tubing would probably be just as good. I.D. is approximately 3/32".
From the top using a drill press (no less), first drilled a 3/32" through hole to meet up with the drain. Then proceeded with a 1/8" drill to give a gentle sliding/press fit for the tube - stopped the drilling just before it went clear through, yielded a bottom-shouldered hole, the tubing just goes in and stops. Cut the tube at 4 mm above top of float bowl rim, it's approximately 36 mm total length.
The first attempt at sealing the tube in the hole used Permatex #1 gasket sealant. According to the package, it was supposed to cure hard and be gasoline resistant. Didn't work very well and one of the bowls started leaking. So now (May '08) I'm using epoxy, so far so good. Cleared out excess glue with a 3/32" drill.
Reassembled the RHS float bowl to the carb body carefully, the tube didn't seem to bind on anything.
April '08, took a test ride with the one modified float bowl, no leaking, seems to work.
Removed the other three float bowls to complete the modifications. If you're patient, it's possible to remove the float bowls with the carbs on the bike. While in there, I changed the main jets from 125 to 120 and replaced the bowl screws with stainless button-head allen bolts.