Owing to their ability to concentrate light on nanometer scales, plasmonic surface structures are ideally suited for on-chip functionalization with nonlinear or gain materials. However, achieving a high effective quantum yield across a surface requires not only strong light localization but also control over losses. Here, we report on a particular class of tunable low-loss metasurfaces featuring dense arrangements of nanometer-sized focal points on a photonic chip with an underlying waveguide channel. Guided within the plane, the photonic wave evanescently couples to the nanogaps, concentrating light in a lattice of hot-spots. In studying the energy transfer between photonic and plasmonic channels of single trimer molecules and triangular nanogap tilings in dependence on element size, we identify different regimes of operation. We show that the product of field enhancement, propagation length, and element size is close to constant in both the radiative and subwavelength regimes, opening pathways for device designs that combine high-field enhancements with large propagation lengths.