correct that AS is not supported here, but at least in normal openvpn this is solved in your NAT rule... I dont know if AS configures NAT for you.
for a basic iptables NAT where 10.8.0.x is the vpn network:
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
to choose what IP address to NAT as, you can use
iptables -t nat -I POSTROUTING -o eth0 -j SNAT --to <IP ADDRESS>