I did exactly what you are thinking about doing.... I put 16oz. of espresso into my priming tank, along with the priming sugar. If you wanted variety, I would suggest bottling half the brew as is, then add 8oz. of espresso to the last half of the brew. That way you get two distinct flavors with the same batch. I think it was my best so far.
NRS
Planned: Vanilla Porter,
Brewing:HolidayAle Bottled: Bavarian Hefeweizen, Belgian Triple, Lemon Wheat
Gone but not forgotten: Brown Ale, Holiday Ale, Raspberry Wheat, Espresso