As I said, a compromise here would be to have a client security certification program, where no other clients outside it would be able to use Signal.

You mean running a trojan “as a mean of security”, similar to anticheats? Are you sure this is a good idea?

Or if by “program” you mean having some allowed clients as opposite to only the official one allowed, it’s a social thing, not a technical one. So it still won’t prevent anyone from connecting with another client.