I have a Jenkins Master running on serverA in vpcA.

I have a server serverB that leaves in vpcB that I would like to register as a Jenkins slave.

I have a jumpbox serverC that is accessible from anywhere.

I can connect from serverA to serverB if I proxy through serverC first.

I can connect with no problems with an ssh config file on the master similar to:
 

Host slave
  User andrei
  ProxyCommand ssh andrei@gateway nc %h %p

I would like to be able to connect serverB as a slave of Jenkins (serverA).
I looked into launching the agent via execution of command on the slave, but that requires the agent.jar to be placed there - is there another way of doing this? Surely if connection from the master to the slave is possible, this could be done automatically, perhaps as part of this plugin?