Is it too late to feed a Japanese maple?
Not sure what you mean by "feed" - plants feed themselves through the process of photosynthesis. If you mean fertilize, why do you think you need to? Is the plant in a container or in the ground? Is it showing signs that it may have a nutrient deficiency?
As to timing of any fertilization, woody plants - should they need it at all - generally do not get fertilized in the middle of the growing season. It tends to be somewhat stressful and can result in growth that does not have sufficient time to harden off properly before cold weather sets in. If required, fertilize either at the start (early spring) of the growing season or in fall after leaf drop or the first hard freeze.