In addition to the regular per-profile dependency tree scans, repoman could figure out the USE flags set by the profile for the package in question and check if they conform to REQUIRED_USE. Otherwise, we warn developers that users may not be able to install the package by default and suggest package.use.
*** Bug 721844 has been marked as a duplicate of this bug. ***
repoman support has been removed per bug 835013. Please file a new bug (or, I suppose, reopen this one) if you feel this check is still applicable to pkgcheck and doesn't already exist.