RMS calculation in frequency domain after windowing
For a reasonably stationary time domain signal you can assume

rms(signal*windows) = rms(signal)*rms(window)


and can estimate the RMS of the un-windowed time signal this way. For a hanning window that means calculate the RMS using Perceval and then multiply with sqrt(8/3). It's not exact but will be fairly close if the signal is stationary with respect to the window length.
If it isn't stationary the interpretation of RMS isn't particularly well defined and it would helpful to learn more about your application and what exactly you are trying to do.