SC2Mapster

I'm also interested in doing this, but not even this simple trigger works. Note that Tower Capture is named "Designate (Unnamed)".

Xel Naga Test 1
    Events
        Unit - Any Unit uses  Designate (Unnamed) at Generic1 - Any stage (Ignore shared abilities)
    Local Variables
    Conditions
    Actions
        UI - Display "xelnaga" for (All players) to Subtitle area

This doesn't work either

Xel Naga Test 2
    Events
        Timer - Every 5.0 seconds of Game Time
    Local Variables
    Conditions
        (Xel'Naga Tower [39.00, 61.00] ability command ((Tower Capture, 0)) is in state Executing) == True
    Actions
        UI - Display "xelnaga" for (All players) to Subtitle area