Is it possible to increase chin length without increasing projection of the chin?

As expert in osseous genioplasties I can tell you this is totally feasible, with the interposition of bone graft to increase the chin without advancing it.

 The jaw is a three-dimensional structure, so any change that it is made with jaw surgery or a chin implant will be a three-dimensional change, not one dimensional. Chin implants are performed under local anesthesia as an outpatient surgical procedure. The chin implant  can be custom carved according to the patient's needs. A sliding genioplasty is performed by an oral surgeon under general anesthesia in hospital setting with an overnight stay. For many examples of chin implants, please see link and the video below

A chin implant allows for improved projection and/or length; this is permanent  and will last you many decades. Please see examples of chin implants below. The benefit of an implant is that you can customize your implant so that it provides you whatever improvement you are seeking.