How to design a secret sharing scheme for matrix-like positioned participants
For an m x n matrix, you can simply create m+n independent sharings (using any n-out-of-n or m-out-of-m secret sharing scheme, such as the XOR scheme), one for each row and for each column.

Each participant will get tow shares, one for his row and one for his column. His total share size is 2s, where s is the length of the secret. Since you can't get share size less than s, this simple scheme is within a factor of 2 of the optimum.