The kinds of exercises Guardian and DFT are describing are exactly the sort of thing I've been doing. I don't use weights in training for force delivery (impact), but rather for hip flexor strength, knee musculature strength and especially balance. My routine in this respects usually consists of strapping two 10lb ankle weights around the kicking leg, performing a slow, balanced chamber, then rotation on the standing leg with rotation of the chambered kicking leg so the bent knee/leg assembly is parallel to the floor, and finally a very slow extension to full—and holding it there for at least 30 seconds. I'm trying to increase the hold, in smooth balance, to two minutes per leg; at that point, I'll probably try to figure out a way to add another ten pounds to the weight assembly. I'm very careful not to do any movements that will involve joint 'shock', and by 'full extension' I mean 99.5% extension; I keep the tiniest bit of bend in the knee at the very end.
My reasons for structuring the exercise this way are that
(i) hip flexors are very difficult to train in the usual 'weight-room' leg exercise regime; you have to actually do full kicks to get them strong enough to do full kicks, and the point is, if you get them strong enough to support a leg which is 20lbs heavier than it was previously, you are going to find supporting the actual weight, with the weights off, a piece of cake;
(ii) the extra weight really, seriously compromises your sense of balance at first, and you have to beef up your balance sense to be able to keep that much weight that far away from your body in consistent balance across a full extension—again, something that will stand you in very good stead when you're just kicking normally; and
(iii) while I agree heartily with people's concerns about joint safety, one thing that I learned when, many decades ago, I was having a problem with an eroding patella—it's a very common condition, apparently—was that heavy stress exercises to build up the knee joint muscle assembly had the effect of stabilizing the patella and keeping it from sliding around again the rest of the joint (which caused a really horrible aching pain, especially at night). My hope—and so far it seems to be working—is that there will be a kind of isometric effect (as DFT alludes to) that feeds not just growth of the hip flexor joint muscles, but of the knee group muscles as well, which will give extra protection to my legs in normal hard kicking.
I appreciate your feedback and cautions on this practice, folks, and believe me I'm not planning on attacking a heavy bag will malice aforethought while wearing leg weights... it's really still an experiment at this point....