Might be worth looking at Linux-mm.org on huge pages. In particular, there's a link to an LWM article on transparent use of huge pages. The "holy grail" is very definitely transparent use, so that whenever possible, all applications gain; anything that makes it easier to move that way is helpful.
One thought; if your mmap parameter is simply a hint that the block will be used in a particular granularity, it's easy to implement. Current mmap sets the parameter to 1 byte (no granularity needed), unless mmaping in hugetlbfs pages, when it sets the parameter to (e.g.) 16M. The kernel then just rounds up to the next highest available page size when possible, or down if not.
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