How do I use a high def shader in low resolution?

I'm creating an original gameboy style game, and want to add the classic gameboy pixel look via a shader. The game is running at the extremely low native gameboy resolution, which works as expected. The problem is, if I try to apply a shader, the shader is also at the same resolution. Most of the time this makes sense, but in this very special case it's not exactly what I want as I need to have lines between each 'pixel'. Is there a way to do this in C3?