Turns out that routing was very simple. Just obtain a tunnel from a broker (I use BT Exact).
Once you have a tunnel, you can set it up like so in baselayout
iptunnel_sit1="mode sit remote $REMOTE_IPV4 ttl 64" config_sit1=( "$TUNNEL_IPV6" ) routes_sit1=( "2000::/3" )
Then setup radvd for your allocation block (don't forget to advertise the router this time) and you're good to go 😃