[firedrake] Prescribe pressure at a corner for Darcy's equation

Hi all, I am trying to simulate the 2D benchmark Elder problem, which is density-driven fluid flow in porous media. The problem is essentially Darcy's equation where no flow boundary conditions are enforced on all four sides (let's suppose the computational domain is 600m by 150m). To ensure uniqueness, pressure is prescribed at the top left and top right corners. Suppose I want to use the RT0 formulation. If my pressure is non-zero, how would I prescribe if I were to use one of the built-in Mesh functions (as opposed to GMSH)? I know for RT0, the pressure bc is prescribed weakly but how do I literally only have the two element faces that connect each of the top left and top right corners apply said boundary condition? Thanks, Justin