It's doable with the camera key. Install shortcutd and set the trigger to a custom command.

The call you'll want to run is;

dbus-send --system --print-reply --dest=com.nokia.csd.Call /com/nokia/csd/call/1 com.nokia.csd.Call.Instance.Answer

You'll need to intercept the camera button for a full depress.

It would be much nicer if it was possible to pickup/hangup using the power button since it's a single stage button but from sniffing dbus, nothing seems to trigger when it's pressed unfortunately.