If you simply want the SAME HDMI output signal duplicated across multiple HDMI-input display devices, then they make HDMI splitter / HDMI replicator devices for driving video walls, etc.
But if you're talking about doing MST (multi-stream transport) like DisplayPort does, sorry, I think that you're out of luck. HDMI has no such capabilities.
Edit: If you simply need an additional HDMI output - they make inexpensive ($10-15) converters between DisplayPort outputs and HDMI, you could get one of those and use it with one of your DisplayPort outputs on your card.