I don't know of anyone who has made a mini-chiller as you described. I think it would work, but probably not as well as you anticipate. Much depends on the temperature of your cooling water and the wort flow rate. Cooling could not only take a little longer as you mentioned, but it could take a lot longer as well. Be advised that most soft copper tubing nominal sizing refers to the outside diameter. It doesn't look like you would have much clearance between the two for sufficient cooling water flow. I'm guessing on that though, so I could well be wrong. Here's some stolen info on ID's vs OD's for the tubing sizes specified: (keep in mind that not all soft copper tubing has these same dimensions as there are several different types available IIRC):
Size (O.D.) Size (I.D.)
You could easily do some trial test runs without much cost.