The simple answer to your question is yes, HA fillers can be used, and have been used many years already for augmenting hollow temples. Although the use of autologous fat would seem reasonable, there are a number of reasons I personally stopped performing fat injections many years ago for any facial augmentation purposes. For one, despite the promise that the fat will remain permanently, most of the fat, separated from its original donor blood supply, does not survive in the recipient location. What's more, the fat that does survive retains the characteristics of the area from which it is taken (which is usually the belly, hips or thigh regions). And this means that when with aging the belly fat develops midriff bulge, the fat in the temples can do so, as well. All this is besides being an expensive, two-step, invasive procedure that engenders real post-treatment downtime. What's more, if for any reason you don't like the results, there is no enzyme that easily dissolves the fat, the way hyaluronidase dissolves HA fillers. You would be wise to consult with an experienced, board certified cosmetic physician injector. Hope this helps and best of luck.