Regarding the name - how about adopting MAP_EXCL? It was introduced in FreeBSD,
and seems to do exactly this; quoting mmap(2):
MAP_FIXED Do not permit the system to select a different address
than the one specified. If the specified address
cannot be used, mmap() will fail. If MAP_FIXED is
specified, addr must be a multiple of the page size.
If MAP_EXCL is not specified, a successful MAP_FIXED
request replaces any previous mappings for the
process' pages in the range from addr to addr + len.
In contrast, if MAP_EXCL is specified, the request
will fail if a mapping already exists within the
range.
