[kpreid]

The cable is the least likely part to be the source of the problem. The data lines are inside a shield, and in USB they are balanced (“D+” and “D-”). Both of these act to prevent radiation of the signals.

If a keyboard radiates it is likely to be either from the unshielded, unbalanced matrix wires going to the keyswitches, or leakage from the controller going onto the outside of the shield. (I think the latter could be reduced by using (more/bigger) decoupling capacitors, i.e. shorting out the RF.)

Coiling a wire does not generally make it a more effective antenna; it may or may not make it less effective depending on the circumstances. (The reason some antennas are coiled is to get the same length of conductor into a smaller space.)