A suggestion:

Install LimeWire on the gateway Linux box and set these in the limewire.props file:
FORCE_SUPERNODE_MODE=true
EVER_SUPERNODE_CAPABLE=true
DISABLE_SUPERNODE_MODE=false

On the computers in the internal net delete all of the existing quick connect hosts in the LimeWire options and add the IP of the gateway. For the Keep approx. connections up in the Connections tab set this value to 1.

See if this works. Firewall settings on the client machines or the gateway might also need to be adjusted but that is more difficult for me to picture what the changes would be right now.