Astilbe 'White Gloria' and Echinacea 'White Swan' are the first to pop into my head that meet your height and color requirements. There's probably enough sun in that spot for the Echinacea and not too much for the astilbe. Both are low maintenance, long-lived perennials. White gay feather/liatris spicata is another.

What type of Japanese maple is it - tall, larger-leafed one (e.g. "Bloodgood"), or a shorter laceleaf type?

If it's a shorter laceleaf type, I wouldn't go with a 2-3' perennial in front of it - it's going to really take away from the beauty of the Japanese maple. Also to my eye, too much in the way of flowers/color business takes away from the particular effect laceleaf maples give - I prefer to concentrate on foliage effects instead.

I love bergenia in combination with laceleaf Japanese maples - the foliage contrast is oustanding, they set each other off beautifully. There's also hosta, if you'd like to go for interesting foliage effects; shorter blue-leafed hostas would look particularly nice in combination with a laceleaf maple. Brunnera is another lovely perennial that looks great with laceleaf Japanese maples.

If it's a tall "Bloodgood" type, you have lots of choices, it's a whole other design ballgame there. The astilbe rec above is a good one. You could go with Japanese anemones for a fall bloomer.

Its a laceleaf maple about 5 feet tall. I have astilbe in other locations so would prefer not to have it here. The thing is that anything I plant in this area has to have flowers that stick up from behind the boxwood hedge so I'm not sure hostas work as the foliage would be obscured by the hedge. You may be right that something 3 feet is too tall and will take away from the maple. Echinacea is probably too tall so I'm guessing I probably need something about a foot and half tall.

You could look for a plant with basal leaves that sends out a bloom stalk either perhaps higher than the maple to draw the eye upward. You could of course find some of the dwarf conifers or varigated boxwoods to provide color and structure to the area with a low ground cover over the entire area. I like thyme or strawberries for the ground cover. Because they fill in fairly quickly and provide useful trimmings.

This site has a neat plant selector that helps with garden planning. You can put in your zone, light and water requirements for the spot you're planting and it narrows down your choices for you.

Here is a link that might be useful: Plant selector