Forthcoming large-scale structure surveys will soon allow us to access Hubble-scale perturbation modes (with wavenumbers k ~ H) at late times. If a modified gravity theory is responsible for cosmic acceleration, the Hubble scale is a natural regime for deviations from General Relativity (GR) to become manifest. We consider the impact of modified gravity (MG) on the set of horizon-scale relativistic corrections to large-scale structure observables that are known in GR, adopting a model-independent ethos that relies only on generic physical properties of MG theories. We assess the detectability of ultra-large scale deviations from GR with future experiments (e.g. Euclid and the SKA), and compare them with existing observables operating in the quasi-static, sub-horizon regime.