Decomposing a waveform in LTspice to view only the transient component
In order to subtract the steady-state response from the transient waveform, you will always need prior knowledge about the steady-state itself. The simulator has no way of looking in the future for transient simulations.

The easiest way is probably determining the output amplitude and phase (because the system is LTI) and then subtracting it from the output. This amplitude and phase can be found by executing an AC analysis.