Some kind of code...

I found this Ema (votive tablet) in the grounds of Meiji Jingu last month. A month of playing around with various cyphers has yeilded no results. It's not a straightforward substitution code so frequency analysis doesn't work, I've tried substitutions with an increasing or decreasing offset, backwards and forwards alphabets...

Obviously the person who wrote it wants to keep it a secret so we probably shouldn't probe further but...anybody got any ideas?