I work for Intel Customer Support but I am not an expert on vPRO* or AMT* so I would recommend that you post this question to: Intel® vPro™ Expert Center they would be able to help you better than in this forum.
Just one more additional comment: Are you using any sort of management console? Intel AMT can use a virtual serial port but it is virtual and the OS sees it as a serial port, a management console securely connected to Intel® AMT using the Ethernet port so that it can send and receive data to this virtual serial port. If no management software is connected, the data sent to the serial port is simply lost.
thanks for the hint, I posted the question there.
The issue is not that the output is lost, I have a terminal connected via AMT. The issue is that the output command hangs, presumably because it's waiting for an IRQ that never happens.