Marek,
First of all, "-g" is just flooded through the network. We also want some kind of "dyngw"-plugin but with a better approach: By using tunnels to the gateway batman is in a far better situation than OLSR. The "is internet really available" check can be done on the client side and is therefore much more
it's about the gateway himself (not a client-of-the-gateway). A gateway router (has to/)should check his default route and change his role to 'ordinary-client' => switch working state from -g to -r/-p Mode. May think about NATed LAN-clients, too...
The "working-tunnel-check" is good tool against 'vandalism' or mis-configuration in the hand of a gateway-client, but it's no protection for the gateway against vandalism (i.e.) of his ISP...
Lui (stucking just in the mentioned situation)