Yes, most hyaluronic acid based fillers would work nicely there. You will need to find a highly experienced injector as injections in some area of the forehead carry higher risk and should not be performed by basic level injectors. With that said, I remove alot of forehead lipomas/ cysts and it is uncommon to wind up with a "dent" after removal as long as any cuts in the frontalis muscle are carefully repaired.
Stephen Weber MD, FACS
Denver Facial Plastic Surgeon
Fillers are great way to fill in any volume related defects or shadows in the face. I have used fillers to correct surgical scars or indentation as related to removal of masses in the face or other parts of the body. If were looking for something a little more permanent and longer-term, then I would also consider using things like fat grafting. I would make sure that you go to someone very experienced in this area, as the fat has to be treated very carefully in order for the grafts to survive.
If you were using fillers, I would recommend using things like Restylanel or Juvederrm as an initial one. These are reversible, should you not like the result. The only downside of using fillers, is their longevity. You may get a year or more longevity, but you may need to touch this up in the future.
Most lipomas are deep to the forehead muscles (frontalis). I generally prefer to remove these endoscopically in patients that are not balding. This usually places a couple of incisions (no head shaving) behind the hair line to leave to visible scars. In general, lipomas are encapsulated and do not leave a soft tissue defect as long as the surgical approach respects the correct planes. In the unfortunate event of being left with a divot (this can happen when the lipoma splays the frontalis muscle or the surgical approach disrupts the muscle without proper reconstruction), I would echo the other recommendations for hyaluronic acid fillers or autologous fat micro injections.