We discuss the use of generalized, symmetry-adapted, imaginary-time correlation functions to study the rotational spectrum of doped helium clusters within the frame of the reptation quantum Monte Carlo method. Analysis of these correlation functions allows one to enhance the computational efficiency in the calculation of weak spectral features, as well as to get a qualitative insight into the nature of the different lines. The usefulness of this approach is demonstrated by a study of the He-CO binary complex, used as a benchmark case, as well as by preliminary results for the satellite band recently observed in the IR spectrum of the CO2 molecule solvated in He nanodroplets.