It's a terrible idea in the first place. You're asking for a world of trouble with joint issues and the like. The only appropriated weighted energy systems work might be weighted incline walking. You can get the cardiovascular benefit you need by normal interval work and hill training with a much lower risk of injury. And your muscles will get woked out fine from you weight workouts.
I agree with rsg, the X-Vest is the way to go. I do almost everything with it on. Mow the lawn, walk the dog, GPP work, shadow box, kettlebell workouts, clean the house, sprint work at the track. I love the thing. Start out low and gradually add weight, 1-2 pounds every 2 weeks or so. Coach Davies site has the best price if you care.
I've done that. at my gym they have very thick heavy chains, each one wieghs 25lbs. i waer the 2 chains across my torso in X formation kinda like Rambo wearing ammo.
I sometimes do whatever standing exercise I have planned with the chains on for added umpfff, such as bbell curls, oh press, heavy bag punching. or I just do light jogging with chains on gives you a great feeling, and nice way to mix it up a bit!!
if you really want to use extra load for your hill sprints, try to attach a heavy enough tire on the rope to your belt. This is probably the only way of doing so without exessice wearing out your knees and back.
-Do it only as a part of some periodised routine, -1xweek is more than enough -build repetitions up gradually from 3-6 max! -allow full recovery between repetitions -do NOT do it for too long -after 4 weeks change it for some streight hill sprints or 400m's after that to continue your training
I do not know what your sport is... however, re back pack with dumbbells - I made some folks WALK with heavy backpacks (small female @ 52 kg/110 lb with 20-22/45-50lb load on her back) for couple of hours as a part of preperation for Maraton des Sables where they need to carry their pack with them. I reckon it went very well