Ryser's maximum permanent conjecture for regular (0,1)-matrices

Let $\mathscr{U}(v,k)$ be the set of all $(0,1)$-matrices of order $v$ with exactly $k$ 1's in each row and column. Let $J$ be the all-1 matrix of order $k$, and suppose that $k$ divides $v$. Let $J^*$ be the direct sum of $v/k$ copies of $J$. Then $J^*\in\mathscr{U}(v,k)$.

Ryser's conjecture. The permanent of $J^*$ is maximal in $\mathscr{U}(v,k)$.

This is a classical extremal problem for permanents of regular $(0,1)$-matrices. The supplied text gives no resolution status, so the conjecture is recorded as open.