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Light can be confined transversely and delivered axially in a waveguide. However, waveguides are lossy static structures whose modal characteristics are fundamentally determined by the boundary conditions, and thus cannot be readily changed post-fabrication. Here we show that unpatterned planar optical films can be exploited for low-loss two-dimensional waveguiding by using `space-time' wave packets, which are the unique family of one-dimensional propagation-invariant pulsed optical beams. We observe `hybrid guided' space-time modes that are index-guided in one transverse dimension in the film and localized along the unbounded transverse dimension via the intrinsic spatio-temporal structure of the field. We demonstrate that these field configurations enable overriding the boundary conditions by varying post-fabrication the group index of the fundamental mode in a 2-\(\mu\)m-thick, 25-mm-long silica film, which is achieved by modifying the field's spatio-temporal structure along the unbounded dimension. Tunability of the group index over an unprecedented range from 1.26 to 1.77 around the planar-waveguide value of 1.47 is verified - while maintaining a spectrally flat zero-dispersion profile. Our work paves the way to to the utilization of space-time wave packets in on-chip photonic platforms, and may enable new phase-matching strategies that circumvent the restrictions due to intrinsic material properties.