Your question:
If a board with unmovable squares, such as Chessence, has a Cannon on a1, and an enemy piece on a3, can the Cannon jump over the unmovable square (a2) and capture the target?
Relevant background information from Wikipedia on hoppers:
A hopper is a piece that moves by jumping over another piece (called a hurdle). The hurdle can be any piece of any color. Unless it can jump over a piece, a hopper cannot move.
Relevant background information from Wikipedia on cannons:
In xiangqi (Chinese chess), the cannon captures as a hopper along rook lines (when not capturing, it is a (1,0)-rider which cannot jump, the same as a rook); in janggi (Korean chess), the cannon is a hopper along rook lines when moving or capturing.
Relevant background information from Wikipedia on Chessence:
The board is a 6×9 rectangle of squares with eight squares missing. [...] A man may not move to, nor through, a non-existent (blackened-out) square; except that a man moving as a knight may jump over a non-existent square.
So the question is really whether a hopper (such as a cannon) can use a square that is not part of the board as if it was a square of the board that is occupied by a hurdle (a piece of any color).
Phrased like that the obvious answer seems that no, a square that is not part of the board is not like a non-empty square. However if we imagine that all unavailable squares are instead occupied by neutral (belonging to neither player) immovable pieces (obstacles) and we also stipulate that these pieces cannot be captured by either player or by any piece, then none of the rules of Chessence require any change. Both a void and an obstacle stop the movement of a rider such as a rook or bishop, so in that respect a non-board void square is very much like a neutrally-occupied square. Prior art for this interpretation exists as the obstacles in the Shuuro chess variant, although knights are granted the power to sit on top of such obstacles. I am not aware of any chess variant that combines hoppers with obstacles, though.
In conclusion, given that voids and obstacles are interchangeable for most rules, there is an argument to be made for either allowing or disallowing using voids as hurdles. However, games that use the language of voids should be expected to default to disallowing, and games that use the language of obstacles do not have a clear default. So be sure to talk to your opponent about the rules that you are going to play by!
