How to keep a chocolate fondue in a liquid form?
You can either maintain the heat level through a heat source served with the food at the table or you can add a neutral-tasting oil to the chocolate mixture such as canola(rape seed) or vegetable oil.  Adding another fat will also help in preventing scorching of the chocolate.

Also the addition of some dairy to create a chocolate "sauce" can be a solution as well, but remember that fondue is about heating(and even cooking) food.  A cold bowl of chocolate, even if it is liquid at room temperature, probably isn't going to be as enjoyable for your guests.