You can submit your flight plan while offline using the flight plan section of my.vatsim.net and then when you connect at the gate, ATC will have your flight plan and you would proceed by talking to whichever ATCO is available at your location. If there is none, or ATC disconnects, just continue with your planned route, broadcasting on UNICOM as necessary. You can't reliably anticipate when ATC might become available but as you know, VATSpy will show you existing cover.