My first thought on unix was mmap() as well, but one of the requirements was that it doesn't "look like" the process is using that memory, which I interpret as "I don't want to allocate this as virtual memory." I don't think mmap() allows that, as its entire purpose is to allocate virtual memory and return a pointer to that memory space.

Personally, my feeling is: why does it matter if it "looks like" a process is using a lot of memory? That is, why does it matter if a process allocates a lot of virtual memory up front? It's not consuming physical memory, it's just updating some bookkeeping in the kernel. I know people feel uneasy about seeing large values for virtual memory, but... they shouldn't.