Could a full 20 millimeters of anterior chin advancement be achieved via a sliding genioplasty followed by a chin implant?

Sliding genioplasty is an extreme surgery that I don’t recommend unless you have functional issues. To fix the cosmetic problem of a weak chin, a chin and jaw implant would give you the correction you want. You must find an expert facial plastic surgeon to help you with this. I hope this helps.

That's a lot of projection so it's hard to say without and actual exam.  Best you see multiple experienced Oral Surgeons and Facial plastic surgeons before proceeding.Chase Lay MD

 Hi, I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  Non smiling photos of your face from the front and side would help in the evaluation.  20 mm's of forward projection is very high, so it would be interesting to see how weak your chin appears in the photos.  The typical forward projection is far less when augmenting the chin. When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  In my opinion, you are a good candidate for chin implant surgery.  
Hope this helps.