I found a trick online for using the shader to tile arbitrarily by scalar constants, so the map turns itself into a 3x3 copy. From there you can teleport the camera when it gets too close to the middle image's edge (to the other side of middle image), and every object (including collision & cursor collision) needs 9 sprites offset into a grid with known spacing so you can see them "wrap" around. You'll also need to set up a Lua module with shared functions for converting between object coords and map coords. In short, the middle 3x3 is where everything (including physics) actually happens and the player is constrained to it. Near edges and corners, you're seeing shallow copies precisely where the real objects would be.
It's all an illusion, but I think it came together shockingly well.