I'm not an expert with the timer functions as I don't use any of them.. that said, from reading the manual, it doesn't look like those functions will cut out your grid power.. From what I can tell, they cut out all power by putting the SI into standby mode.
Two work arounds might be available..
1) Your suggestion to put a contactor on the grid lines
2) (I say this with caution because you'll have to research it) You might be able to trick the SI into thinking the grid is a generator. You DO NOT want to make it think the generator is the grid because of all the tight frequency/voltage tolerances, plus the reverse current issue since pumping current back into your genset would be very bad..
But I don't see any obvious issues with making the SI think the grid is a generator.. in which case, you could then play with the generator timer functions and set the timers so the generator (the grid actually), is only allowed to be used during certain times that you specify.
I'm not sure if #2 will work, its just an idea, but I do know you can make the Sunny Island think the grid is just a generator because I've done it. The Sunny Islands must have been thinking "Wow, that's one hell of a good generator this guy has!" LOL