Bit late this year for summer bedding, but bedding plants which tolerate shade well are fuchsia, buzy lizzies, begonias, lobelia, pansies.

As Acers prefer Ericaceous compost as you probably know :) , a pot in semi-shade would be a good place to grow them. I also have an Enkianthus (lovely autumn colour) in a pot, and a Pieris Forest Flame in semi-shade.