An optimal control problem when controls act on the
boundary can also be understood as a variational principle under differential
constraints and no restrictions on boundary and/or initial values. From this
perspective, some existence theorems can be proved when cost functionals
depend on the gradient of the state. We treat the case of elliptic and
non-elliptic second order state laws only in the two-dimensional
situation. Our results are based on deep facts about
gradient Young measures.