How to prevent a baker's chocolate mixture from resolidifying so that it can be used in beverages?
I'm not sure if this is what you're looking for, but what came to my mind was making a ganache - pouring heated cream over chopped chocolate and then stirring until the chocolate is melted.  By adjusting the ratios of cream to chocolate, you can make a lovely thick chocolate syrup that works well for stirring into beverages.

There are 4 standard ratios of chocolate and cream, for different purposes:

2:1 (chocolate:cream) is very firm and holds its shape, good for uncoated truffles.

3:2 is firm but slightly softer; good for coated chocolates and cake icings.

1:1 is soft and smooth; good for tart and cake fillings, some cake icings, and spreads

1:2 is pourable and sauce-like - the obvious choice for sauces.



(from a Chowhound article on ganache)