looking at the op-code in the World of Minecraft.exe see the following below:

* String: "-Xms"

0040356B: BF4D734000           mov edi, 0040734D
00403570: 890424               mov [esp], eax
00403573: E888200000           call KERNEL32.GlobalMemoryStatus

* String: "-Xmx"

0040358C: BF52734000           mov edi, 00407352
00403591: 894C240C             mov [esp+0C], ecx
00403595: 89542408             mov [esp+08], edx

i will confirm if indeed these are pointer to command$

then it may be possible to do World of Minecraft.exe -Xms800m

i'd say 70-30% against it working.

confirmed only way to allocate extra allocate memory would be via the java code.