"The --enable-alsa09-mmap option to ./configure turns on memory-mapped I/O. Note this is an experimental feature and causes some problems with some sound card drivers and also users of the dmix software mixer." A USE flag like mmap for --enable-alsa09-mmap would be nice. As it is, to enable mmap, the ebuild must be manually edited and emerged.
libao-0.8.4-r1 now has this option.