RESTRICT="network-sandbox" is supported: http://gitweb.gentoo.org/proj/portage.git/commit/?id=d56707b9d3451810562fc1e763d4916ab02dd54e
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.