Prime model with mock data, good idea or not?
This is not the answer you'll want to hear, but I would say, "no." Priming is not a good idea. A model is only as good as its inputs. If you're making up the inputs yourself, then your model isn't learning real patterns and you might as well just hard code a set of rules that represent how you would have created the primed data. 

A fancier way might be to make a bayesian model where you create priors based on your assumptions. The bayesian model would then evolve as real data becomes available.