Power exhaust is a critical challenge for spherical tokamak reactors, making the design, optimisation and control of advanced divertor configurations crucial. These tasks are greatly simplified if the poloidal magnetic fields in the core and divertor regions can be varied independently. We present a novel method which fixes the core plasma equilibrium whilst altering the divertor geometry, using vacuum spherical harmonic constraints. This has the advantage that it avoids iterative solution of the Grad-Shafranov equation, making it easy to use, rapid and reliable. By comparing a large number of MAST-U equilibrium reconstructions against their approximations using spherical harmonics, we show that a small number ($sim4$) of harmonics is sufficient to closely reproduce the plasma boundary shape. When augmented with divertor geometry constraints, this method gives great flexibility in the creation of new exhaust configurations. We discuss how this approach would benefit applications in feed-forward scenario design, coilset optimisation, and real-time feedback control.