Can I change Synapse shortcut to Super/Windows key alone?

I know this is a little old now but thought solution by be useful anyway.

First remove any super key bindings e.g. <super>+p is a common one for screen settings

Then close Synapse

In ~/.config/hotkeys/synapse.hotkeys

change

Signature=<Control>space

to

Signature=Super_L

Relaunch Synapse

In later versions of synapse installed from the launchpad ppa the setting is in .config/synapse/config.json. Edit it while synapse is closed to specify Super_L as activation key.

{
  "ui" : {
    "shortcuts" : {
      "activate" : "Super_L",
      ...
    }
  }
}

See https://launchpad.net/~synapse-core/+archive/ubuntu/testing.

See better answer below this one.

You must bind the action to the Super's scancode or keycode; normal hotkeys record the state of the modifier keys and throw away their keypress events; so you never see something that reacts to just pressing Alt, but instead to the A key plus the Alt key's recorded state.

The hotkey library in use is libgtkhotkey (0.2 on 12.04) and it doesn't support using the key press event from modifier's. You'd have to write your own patch to catch that and perhaps support such a thing in the signature format.

All tests with the existing signature format failed to produce anything but an error.