here are three fireplaces that I did that may provide you with some ideas.
The first one I anchored expanded metal lath into the brick. coated with two layers of fibered cement then two layers of American Clay's clay plaster with slate tile for the hearth. This method may be overkill.
The next one I troweled on two coats of a mixture of setting type joint compound and sand followed by a coat of gypsum plaster with the color mixed in which allowed me to get a sort of marbled affect. With the joint compound I find that the Sheetrock brand tends to perform much better than the others that I've tried. It can be applied very thick with no noticeable shrinkage or cracking. I definitely would not use a premixed compound in this way.
The third one I just primed and painted. When going this with this method I will some times use a grinder to clean up unsightly mortar joints. If there are any patches they can be ground to look like the rest of the brick.
Hope this helps
Here is a link to an online album that shows before and afters of these three projects: