(Caveat: This is from memory with a quick peek into the manual. So some reply after trying it might be useful.)
Try fill=yes (*1) for the memory area of MAIN and make HIMEMSEG overwrite the area you added:
ZP: file = "", start = $0002, size = $00FE, define = yes;
LOADADDR: file = %O, start = %S - 2, size = $...
Look at phase/dephase directives (also known as rorg/rend) - e.g. in the vasm manual. The whole code should like:
... (will be assembled for "org RAM_BEGIN", but stored immediately after previous code)