We show, via simulations, that an optical fiber taper waveguide can be an efficient tool for photoluminescence and resonant, extinction spectroscopy of single emitters, such as molecules or colloidal quantum dots, deposited on the surface of a thin dielectric membrane. Placed over a high refractive index membrane, a tapered fiber waveguide induces the formation of hybrid mode waves, akin to dielectric slotted waveguide modes, that provide strong field confinement in the low index gap region. The availability of such gap-confined waves yields potentially high spontaneous emission enhancement factors (≈ 20), fluorescence collection efficiencies (≈ 23%), and transmission extinction (≈ 20%) levels. A factor of two improvement in fluorescence and extinction levels is predicted if the membrane is instead replaced with a suspended channel waveguide. Two configurations, for operation in the visible (≈ 600 nm) and near-infrared (≈ 1300 nm) spectral ranges are evaluated, presenting similar performances.