I2C has no concept of a discovery or identification packet. The host can write any data to any address, and you have to already know in advance what address and data structure you need. The device can't tell you what its data format is.

An I2C transaction begins with a client address, and the host waits for an acknowledge signal that a client has answered the message. That's it, that's the only standard part. From there, the host typically sends a register address and then some number of data bytes, or the client sends back some number of bytes. What those addresses and bytes mean are totally application-specific. There is no standard at all here.

You could absolutely broadcast standard DDC commands to all addresses, but you have no way of knowing what actually happened. You might have turned off one monitor and set another to VGA mode. You might have just crashed some random device on the bus. You have no way to tell.

All your host can possibly know is that there is a device at a given address, and it either did or did not send an acknowledge bit after a command. That's all the information available to you, and it is not enough to do what you suggest