Does anybody know if a part 3D printed in SLA can be polished?
I don't need the entire part polished, just an area that light from an LED will be shining thru.
Yes, it can be polished. Ask the vendor who is printing the part about your options. Mechanical polishing works, but they can also apply a "clear coat" that gives very good optical clarity to the surface and is a cheaper option.
I have also clear coated SLA parts in the past (though it may not help with your original inquiry!) and it also has the advantage of preventing strength degradation and visual degradation of the part long term.