Installing a fixture with no ground, no ceiling box
According to the National Electrical Code, you can replace the fixture without a ground as long as the outlet is GFCI protected.


  National Electrical Code 2014
  
  Chapter 4 Equipment for General Use
  
  Article 410 Luminaires, Lampholders, and Lamps
  
  V. Grounding
  
  410.44 Methods of Grounding Luminaires and equipment
  shall be mechanically connected to an equipment
  grounding conductor as specified in 250.118 and sized in
  accordance with 250.122.
  
  Exception No. 3: Where no equipment grounding conductor
  exists at the outlet, replacement luminaires that are
  GFCI protected shall not be required to be connected to an
  equipment grounding conductor.


As for actually connecting the fixture. If the fixture does have a built in junction box, that is designed for the use. There's no problem making the connections in the box. If not, you'll have to install a box in the ceiling, and make your connections there. If you have to install a junction box in the ceiling, you should be able to install one that will be easily hidden by the fixture.

My other concern is protecting the wires between the AC/MC cable, and the junction box. You might have to install a short bit of conduit, connecting it to the AC/MC cable using an appropriate coupling. The other end of the conduit will then connect to the junction box, and the wires will safely travel inside the conduit.