The person requesting this change controls neither the server nor the client. They control the intermediate network. If they could log the ephemeral keys it would solve the solution, but only the server and client hold that.
As I understand it, if you don't have the server's private key it makes no difference whether a connection uses FS. FS merely means compromise of long-term keys does not compromise past session keys.
Surely for the presence of FS to be relevant, they must already have the server's private key, implying they do have control of the server?
I get that this would be less secure to use in practice because now anyone who gets control over the server or the middleboxes or who can somehow compromise that secure channel between them can get at the shared secret but still, it would preserve forward secrecy. The only caveat is that the secure channel between the server and the middleboxes would also need forward secrecy but I don't really see how that is a problem.
Am I missing something obvious here?