alsa-oss-0.9.0.1_beta10.ebuild should be masked due to the same reasons as the other alsa-0.9's ebuilds are masked. The problem can be solved by add the line: =media-libs/alsa-oss-0.9.0.1_beta10 in package.mask
Can you be more specific on why it should be masked? I was considering unmasking all the alsa-0.9's as creating a link for libasound.so.1 appears to fix the original problems.
Due to the fact that at the moment it was uncertain when the alsa-0.9 drivers would become available for download, i considered this bug-report as usefull since people could accidently try to emerge it and get errors during the emerge-process at the libasound-check as well. Since this problem was already reported in packages.mask for the other 3 alsa-0.9 drivers, this package couldn't be left out for the very same reason. But if you've fixed the alsa-0.9 problem than this bug can also been seen as solved i guess.