The Japanese Garden forum is typically very slow - questions about Japanese maples (and maples in general) are better posted here (although I do check the JG forum from time to time :-))

Where you are located could make a difference. If the tree is planted in full sun, I would look for an alternative to the rocks. They will absorb a great deal of heat and disburse it into the soil and the tree roots. And I would be cautious about what sort of plantings you may choose for underneath the maple. Depending on graft location, you might not see them well anyway and whatever you do select should be shallow rooted and permanent - JM's dislike any disturbance in their root zone and cultivation there can lead to root damage and disease issues. So no annuals, no perennials that need routine division. Some sort of groundcover could work or just a wood-based mulch to finish, like bark or wood chips.