I'm cleaning all my kegs today and had them taken apart. The dip tubes on one were NASTY. Was trying to find a way to clean them, especially the loner beer out tube.
Well...I was also cleaning one of my guns, and I use a Boresnake to clean the barrel. That little light above my head kicked on...
I used a .22 caliber boresnake through both the gas and beer dip tubes and it worked quite well! Naturally if you have one for guns clean it real well before using...or buy a new one dedicated for your kegs. The snake was a little too small but it cleaned out the loose stuff no problem. Tomorrow I'm going to grab either a 6.5mm or 7mm snake which should be perfect. The increased diameter should mean a more thorough cleaning.
If you didn't click on the link they're basically long pieces of fabric with brass brushes attached. When you pull it through a barrel (or dip tube...) the fabric cleans out the loose stuff and the brushes break up the stuck on stuff. For guns I put some barrel cleaning right on the fabric near the brushes and it'll clean just about everything off. The same can be done for cleaning kegs.