Big Target, I see that I'm not the only one to see the possibilities of using the belt loop to hold extra gear.
After a long and tiring search, I finally decided to go with an Ontario PSK as my main belt knife. I put it in a Kydex sheath designed for the Kabar "Shorty" and it fits really well. To the front of the sheath I lashed a compass pouch which holds an Altoids kit, plus 4 tripwire snares, 2 small pieces of fatwood, 50 feet of 20 pound test mono, M-1 Carbine oiler, Lansky ceramic hone, and a BSA Hot Spark rod with striker.
On the back of the sheath, tucked into the paracord holding the compass pouch on, is a 5 inch section of hacksaw blade. I also attached a small pouch to the back of the belt loop that holds an SAS wiresaw, 4x1 inch piece of leather for use as a strop in the field, and a red Photon II LED.
To attach the knife to my belt I used the plastic attachment point alread on the belt loop that is designed for an optional "drop leg" carry. I just used some 1 inch webbing and a Fastex buckle to make my own secondary belt loop. This way the knife rides just low enough to be more easily accesible, and I can "snap" the knife on any belt I wish without disturbing whatever else is already on it.
Keeping another source of emergency supplies attached to your main knife is a great idea. That way you have a complete kit that you can grab and go with. And if you already have a main kit on your belt or in your pockets, the knife kit gives you more gear to use when stuff gets really bad.