Since eventually you will have a pretty dense canopy, you probably want to go with low growing plants. There is a groundcover hosta called lemon lime that might be good. It's cheap and easy to find from mail order, I would think. THere are some gold lamiastrums that are ground covers and easily found at nurseries here, also lamium 'White Nancy' might work.
Kay

I have some variegated Vinca minor growing under mine. It has green and gold leaves, tolerates shade, and is very low growing. See the first few pics in this slide show:
{{gwi:27209}}Japanese Maples - Spring '08

Regards,

K4

I have Sedum repestre 'Angelina' growing under mine.

I've grown Creeping Jenny, black mondo grass, Chrysogonum, partridgeberry, and hosta under an 'Octopus' Jap maple, all in a raised bed with limited root space next to a pond.

All of the above are great possibilities :-) How tall your tree will get and the distance from the canopy to the ground depends on where it was grafted and how it has been trained - some selections will provide greater unobstructed space for plants than others. Whatever you select should have relatively shallow roots and be able to remain in place without need for frequent (or any) division......that's why low growing groundcover-type plants are ideal. Other possibilities could include the Japanese forest grass, Hakonechloa macra 'Aureola', or epimediums. The epis foliage goes through some seasonal color changes, with new growth emerging a bright limey green in spring, often with reddish edging, then maturing to green in summer and with some species, turning a rich coppery red in fall and winter. Plus, there is a wide range of color to the sprays of tiny orchid-like flowers they produce in early spring.