But think about it for a second.
99 times out of 100, a team gives a player an NMC/NTC essentially in exchange for a bit of a lower AAV on their contract. The player gets added security to their life by avoiding being uprooted from their home and family in a way that is beyond their control, and they are usually happy to shave a little off the top in exchange for that. In the interest of cap space, the GM is usually pretty happy to oblige in most cases.
Agents make their money by taking a cut of the contract they helped to secure for their clients. NMCs and NTCs typically drive contract value down from a monetary standpoint. It's pretty unlikely that the push for one of those clauses is being driven by an agent. I'm not 100% certain, but it stands to reason that that kind of request would come from the player. Also, I just don't think the agents have as much power as you might. Most contract negotiations do not stall out, meaning that both sides are typically getting what they want in terms of a signing. The conversation of "my guy wants this clause in his contract or we walk" is probably not one that most players are in a position to have, especially if a team has their signing rights. Also, if a player is that happy to walk (i.e., over a discrepancy in the contract clauses), I would think a GM would be okay trading the rights of that player or executing a sign-and-trade.
But I won't pretend to be an expert on this. I am not. This is just my $0.02. Worth even less