The single-degenerate scenario for Type Ia supernovae (SNe Ia) should yield metal-rich ejecta that enclose some stripped material from the non-degenerate H-rich companion star. We present a large grid of non-local thermodynamic equilibrium steady-state radiative transfer calculations for such hybrid ejecta and provide analytical fits for the Halpha luminosity and equivalent width. Our set of models covers a range of masses for 56Ni and the ejecta, for the stripped material (Mst), and post-explosion epochs from 100 to 300d. The brightness contrast between stripped material and metal-rich ejecta challenges the detection of HI and HeI lines prior to ~100d. Intrinsic and extrinsic optical depth effects also influence the radiation emanating from the stripped material. This inner denser region is marginally thick in the continuum and optically thick in all Balmer lines. The overlying metal-rich ejecta blanket the inner regions, completely below about 5000A, and more sparsely at longer wavelengths. As a consequence, Hbeta should not be observed for all values of Mst through at least 300 days, while Halpha should be observed after ~100d for all Mst >= 0.01Msun. This contrasts with the case of circumstellar (CSM) interaction, not subject to external blanketing, which should produce Halpha and Hbeta lines with a strength dependent primarily on CSM density. We confirm previous analyses that suggest low values of order 0.001Msun for Mst to explain the observations of the two SNe Ia with nebular-phase Halpha detection, in conflict with the much greater stripped mass predicted by hydrodynamical simulations for the single-degenerate scenario. A more likely solution is the double-degenerate scenario, together with CSM interaction, or enclosed material from a tertiary star in a triple system or from a giant planet. [Abridged]
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