the issue with using it as priming sugar: what if the amount of liqueur needed to get the right CO2 volume is way too much/too little chocolate flavor?
I think it'll be easier to ferment it out and priming 'traditionally', unless you are down for some math and taste testing to dial in the final flavor.
Nothing wrong with either approach, just take the one that you think YOU can do the best.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10