You know far more about RVs than I ever will, but it still seems unlikely that a lightweight decoy would have the same ballistic characteristics in the atmosphere as an RV, unless the decoy is small. If that's the case, then the kill vehicle shouldn't have too much trouble discriminating between the actual RV and the warheads.

Playing with the equation you provided and using purely hypothetical numbers:

B = M / (CD * A)

RV:
M = 400 kg
Cd = 0.5
A = 1 sq meter

Beta = 400 / .5 = 800

To get the same Beta with a lightweight decoy:

Decoy:

M = 2 KG
(Cd * A) = M / B

(Cd * A) = 2 / 800 = 0.0025

A = 0.0025 / Cd

The only way to increase the area is to have a very low Cd. If the Cd was 0.01 (a very low number), then the area would be .25 sq meter. I don't know if you can get a decoy with a Cd that low.

On the other hand, you can increase the size and Cd if you increase the decoy mass, but eventually, that defeats the purpose.