Using /etc/conf.d/net from baselayout to configure tap0 does not work unless usermode-utilties is emerged. I think this is counterintuitive since anyone who would install openvpn would(?) need to bring up a tap/tun interface and the config script will report "tap0 does not exist" unless tunctl from usermode-utilities is present
baselayout-1.12.0_pre16 will support making tuntap interfaces if you have openvpn installed and not usermode-utilities. This is because openvpn works on many arches whereas usermode-utilities only works on amd64 and x86.
Fixed in baselayout-1.12.0_pre16