Why? In today's world, writing to an mmap-ed file most certainly doesn't hit the disk for each write. Instead, a block of memory from the buffer cache is used to hold the changes. The only difference is that instead of being backed (VM-wise) by the swap file, the block is backed by the mmap-ed file.
There's no real change here for ZFS, and it's unlikely that anything at the memory cache level even knows about the copy-on-write-ness of ZFS (or even cares).