  We develop a model for the angle-dependent magnetoresistance of
HgBa$_2$CuO$_{4+\delta}$ in the underdoped regime where the Fermi surface is
thought to be reconstructed by an ordered state such as a charge density wave.
We show that such measurements can be employed to unambiguously distinguish the
form of the Fermi surface's interlayer warping, placing severe contraints on
the symmetry and nature of the reconstructing order. We describe experimentally
accessible conditions in which our calculations can be put to the test.
