To be honest, I thought that's how it works right now. At least in Amethyst. This app knows the relays of those I'm subscribed to, and I thought it was looking at them to find out from the message. If this does not work now, then, of course, to be need to dynamically determine the list, and the question is only in time and in the capabilities of developers.
Discussion
Yes, I think too that most clients try to do it that way, with more or less success. But that means that you may not see every reply. But you will not see spam as well.
You don't have to listen to the global feed to get an reply, and you can listen to it from free relays, where there is a lot of spam in the global feed. And to read the global feed, you can listen the global feed of paid relays.
That is, I do not understand why a reply will suddenly be missed (which was sent, for example, to a free relay, where there is a lot of spam)? A free relay is enough for him. In addition, the relays relay each other's network events. Also, if you write a message, your client sends it to both paid and free relay. It seems to me that there should be no problems.
A 22 hours notes and their uptake
