How can I vent a bath fan with no access to exterior?
You need to vent the moisture outside. If you have access to the outer wall cut a hole into the sheathing it can be bigger than the duct. Then, using a masonry bit, drill a single hole through the brick where the center of the duct will be for reference. I suggest not using a hammer drill so as not to bust or crack the brick causing unwanted problems. 

Now go outside using the center hole for reference mark on the brick mark the opening you will need.  Now drill a series of holes alone the mark fairly close together taking your time and be careful--it's better that the opening is smaller than needed because you can always remove more material. 

It's important that you not hurry and drill from the brick facade.  This is because sometimes when drilling thru the back of the brick can break off uneven. Run the duct thru the hole install and seal the vent cover.