I do not see how there is a forced mate here, so I am jumping on the bandwagon of those with the opinion that it cannot be done.
I can see how White can stalemate Black's king, forcing Black to move his pawn down the board, but once it reaches a3 the Black King can just move back and forth between a1 and a2 until White gets tired of shuffling the bishops around and accepts the draw.
With Black's cooperation, if he ever moves his pawn to a2 that will remove the a2 escape square enabling White to replace a bishop on the a1-h8 diagonal and declare mate, but it isn't forced. It requires a blunder for it to work.
The ending position may look something like the following, though with all the bishops they could be spread around a number of different ways by the time this position is reached:
At any given time here, Black has two move choices. One maintains the draw (Ka2) while the other loses the game (a2).