This document describes the seeds as very small, with a primary dispersal means as being wind, but says that, "Once the connection with the plant is severed, the seed has reached its maximum fresh weight. The seed then dries down and enters a quiescent state until proper conditions for germination are met. Seeds of M. sinensis should follow a similar developmental pattern of other C4 grass species."
This pattern suggests that if the seed falls from the flower, then it is mature. Perhaps the simplest method of harvesting the seed on a small scale would be to use the old "bag-it" method of covering the flower with a cloth or fine mesh bag and securing it to the stem so that nothing can fall out. Then you could just check to see when seeds started falling and maybe help things along with a shake of the flower.
However many ornamental grass cultivars grown from collected seed are either not viable or do not "come true" to the parent. Anecdotal reports puts M. sinensis 'gracillimus' grown from collected seed at 50% true to parent. It may not be important in this case, but if you are looking for a uniform crop, it is something to consider.
Here's a wealth of info on this species including this cultivar: Unfortunately there is no mention of determining when seeds are ready to harvest.