BCCI refused the permission in the past but I am not sure BCCI completely banned Indian players from overseas t20 leagues and other other cricket boards too have just kind of need if they want to participate in T20 leagues organized by other boards.
These measures were implemented when N Srinivasan was controlling the BCCI, and I believe that they should remove this ban. It is very unfair from the part of BCCI to ban the Indian players from the other leagues, when all the other boards allow their players to take part in the IPL. Also, two months every year from the international calendar is devoted to the IPL. None of the other boards get such preferential treatment for their leagues. On top of that, despite the revenue redistribution, the BCCI still receives almost 4 times funds from the ICC when compared to some of the smaller boards such as CSA and NZC. The BCCI should not get too greedy and arrogant. If they do that, then the other boards may gang up against them.