The game engine does not support (at the moment) segment prefixes, meaning that
MOV [bx], 0xDEAD
is fine, but
MOV [es:bx], 0xDEAD
throws an unsupported opcode exception.
The way around this is that [bx] references ds by default, so if you edit ds so that it is equal to es, then the code will work, e.g.,
PUSH es ; push the value of es
POP ds ; place it into ds
MOV [bx], 0xDEAD ; write it into [bx], which is [ds:bx] by default