LTSpice - resistor controlled by RMS
RMS is a computed value related to the "output" and therefore it cannot be used to change the value of a component (an input value) because you get a circular problem. Maybe you can take Vn001, and feed it through an RMS to DC converter. Maybe use a multiplier to get v\$^2\$ then low pass filter it then, use a square root circuit: -



Most sims have the multiplier "part" and, with an ideal op-amp (not a TL081 as shown above) you can extract the square root and hence the RMS.