When I run ECA I generally stay away from coffee....I can't handle that much caffeine. I think the average cup of coffee has something like 200mg of caffeine - so just keep it in mind if you're having coffee with your ECA.
It all depends on how you handle caffeine... I made the mistake of drinking two cups of green tea while I was taking an ECA once... NOT fun at all. It's not unsafe, it just depends on your tolerance for the caffeine... I don't know that you'll get any added benefits from the extra caffeine either, but if you like your coffee and can handle it without getting all crazy and shaky, then drink up.