Every $f \in L^\infty([0,1])$ induces a continuous linear functional on BV via $g \mapsto \int f g \mathrm{d}x$. I believe $L^\infty([0,1])$ is also separable in BV$^\ast$, while BV$^\ast$ is not separable, so it cannot be the case the BV$^\ast$ completion of $L^\infty([0,1])$ is all of BV$^\ast$.
Is there a nice characterization of the BV$^\ast$ completion of $L^\infty([0,1])$?