I'm not into making ebuilds yet, so I leave the code up to you. My suggestion is the following: Use the variable PROFILE_ARCH to modify the value of SRC_URI to automatically download the binary tar file for the arch on witch the ebuild is being run. For a list of SRC_URI for some archs, please go to http://setiathome.ssl.berkeley.edu/unix.html
BTW, examples of the use of that variable, can be found on glibc 2.2.5's ebuild
you can do this: SRC_URI="x86? ( pkg to download ) ppc? ( pkg to download ) sparc? ( pkg to download )" and so on and so forth ... if you want support for a specific package file a bug for a specific pkg