I'm just curious, can you expand on that a bit? Yes, it's very simple and a bit hokey; but I don't see any reason it shouldn't work provided the end links are long enough. It only lets you cut the rate; but from what I saw on my TR3A with ADDCO bars front & rear, that's probably the right direction in the rear.
Unless ADDCO has changed it, the adjustable end link kit for the front consists of a pair of bushings for each adaptor plate, a pair for each side of the anti-roll bar, some spacers/distance pieces, cup washers for the bushings, a couple of longish bolts, nuts for the bolts and some little coil springs. I first saw the adjustable link kit when friend got one of those kits to go with his ADDCO bars for his TR250. Essentially, while the set up does allow the bar to be "zero-preloaded," it left a gap that has to close up to before bringing the bar into play. We installed it anyway, but removed it later to go to a more robust set up. As far as cutting the rate of the bar, once the bar is brought into play I don't really see that since the rate for the bar is defined by the length of the bar between the arms, the effective length of the arms, the diameter of the bar and the modulus of elasticity for the bar material.
For the rear of the IRS TR series cars there was no adjustable end link kit offered, just two washers per side, one rather large (2 1/4" OD, 3/8" ID, 1/8" thick per Kastner callout and that sounds about right for what ADDCO supplies), the other not as large (1 3/4" OD, balance same as other washer) that together make the reaction plate for the bar/end links with both fitted into the spring pan of the trailing arm, four cup washers for the upper most and lower most bushings, four link bushings per side plus the hardware for attaching the bar to the frame. The washers as supplied by ADDCO are loose and really should be welded together to provide a more stable reaction plate for the anti-roll bar. Note that other than the washers for the reaction plate, the balance of the Kastner end link set up for the rear is very different from the ADDCO set up but probably not as street friendly since it would cost more ground clearance than the ADDCO set up.