Separation of a many-body system into a primary system plus a bath of background modes enables approximate calculation of electronic absorption spectra and zero-temperature resonance Raman scattering cross sections in cases where there is nonadiabatic coupling between two or more Born-Oppenheimer excited-state potential surfaces. In particular, the low-resolution optical line shape theory recently developed to describe curve-crossing phenomena [D. G Evans and R. D. Coalson, J. Chem. Phys. 99, 6264 (1993)] is extended to systems where there is a primary nuclear coordinate mode that is characterized by large excited-state displacements and an ensemble of weakly displaced bath modes. The accuracy of the resulting approximation scheme is illustrated using the spectroscopic spin-boson model, in which all surfaces are harmonic, and path integral techniques can be used to obtain exact results. Application to more complicated systems is discussed.