Yes, that is allowed so long as you pack out your trash. I've eaten a quiet lunch here many times; great place for a small picnic :)
I have never heard or known of anyone expressing the desire to picnic in a cemetary, nor do I know for certain if the rules allow for this. As you enter the cemetary at the main entrance point on SE 28, you will notice a big black and white metal sign which lists the rules. If within the boundary of the rules it does not state you cannot picnic in the area, I see no reason why that would be a problem to have a small, quiet gathering just as long as you are very respectful and clean up well after yourselves.
How is eating in a cemetery being respectful? Unless you're leaving food FOR the dead--like the Chinese do for their loved ones--you should not have a picnic...
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