Actually Pikes' post does apply to the second scenario.
...and to refer to Broker all communications received in any form from real estate brokers...
If the company wanting to put the sign in the ground is a real estate brokerage (which they would have to be in order to represent a buyer in the transaction) then the seller is to refer all "communications" by that real estate broker, to their own listing agent.
Creating an agreement whereby the secondary company is allowed to place a sign in the yard and advertise for buyers would take "communication" which should have been referred to the listing agent.
Whether it's legal or not is for an attorney to decide...but based on my understanding of english, it would not be allowed based on the portion of the listing agreement that Pikes presented.