540 Mini-T's and Micro RS4s are possible due to the motor being positioned transversly (sideways) at the rear of the chassis.
This allows you to design custom motor plates and keep the big 540 can away from the rear axle. You achieve gear meshing by using oversized pinions which the 540 class motors are capable of driving due to the torque they produce.
The RC18T (like the M18) has a longitudal motor configuration.
This presents some major engineering challenges when it comes to squeezing a 540 motor into the chassis. Another factor is the weight gain on the side of the chassis where the motor is positioned.
It could be done, no one has done it as far as I am aware. A Mamba25 (5400kV/6800kV) Brushless would be a better solution.
Bigger isn't always better, the power produced by a 540 tends to wear Micro (1/18th-1/20th scale) cars and trucks out quickly unless some major modifications are done.
AktionDan designed a (Micro RS4) 540 conversion chassis for that specific reason.
His new
"Sleeper 480" conversion offers more speed than a 540 conversion offered.
I wouldn't contemplate doing a 540 conversion on my RC18T and I've decided it needs a lot of hop ups before I can install the Mamba25 Brushless I have for it.