The Reserve is totally under army and presidential control at all times.
whereas the state governors have some power of national guard for emergencies within their own states if they are not otherwise mobilized for national defense
Warner Defense Authorization Act of 2007 H.R. 5122
Federal law was changed so that the Governor of a state is no longer the sole commander in chief of the National Guard during emergencies within the state. The President of the United States will now be able to take control of a states National Guard units without the governors consent. [1] In a letter to Congress all 50 governors opposed the increase in power of the president over the National Guard.(wikipedia, take always with salt)
Well, according to Article I, Sec. 8, the Congress has the following powers: "To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;"
So, I guess it would depend on the nature of the emergency, if this was legal. It would have to involve executing Laws, suppressing insurrection, or repelling invasion.