No, as you mentioned, 406link is a commercial hack that detects a PLB's "test" message and uses that to trigger a pre-programmed "I'm ok" message. But that's not what I'm referring to.
By one-way communication, I was referring to the default "I'm in distress, here's my coordinates, send help" message, which is the only message the system can officially transmit. There's no way to carry on a two-way conversation to collect further details, or even a mechanism to acknowledge the message - though due to a very robust system design, it's very unlikely the message wouldn't get received.
It's interesting because it was never intended to work like that, it's basically just a hack that uses a test signal to dispatch an "I'm ok" message