@dadcue I know seeking an answer can be tough when your doctors don't really have enough information on how to prevent heterotopic ossification. I have been looking for research papers trying to find information that could help. Here are some of my thoughts, and perhaps it may lead to asking specific questions of your medical providers. I am not a trained medical professional, but I have knowledge because of a background in biology, having worked in scientific research, and what I have learned as a patient from my experiences with surgeries and physical therapy. I will list some links at the end where you can read about research involving HO.

Generally, I've learned that there is an inflammatory process that happens during the formation of extra bone as you've also said, but a few other things also caught my attention. Bone formation happens as a sequence of changes that first create cartilage that is later replaced by bone, and inflammation is part of that process to set the stage for creating new bone.

Sometimes surgeons are using bone morphogenic proteins as a kind of "cement" during surgery and that can be linked to a higher risk for heterotopic ossification (HO) and this is used in some spine surgery.

Spine surgeons also speak about excess pressure causing bone remodeling in the spine, and if I were to take a guess, I would think that scar tissue that forms in the fascial layers because of surgery and the surgical path, could also increase pressure in those tissues and may perhaps increase the risk of HO because the scar tissue may connect directly to the spine via the surgical path. This is why surgeons want knee replacement patients to begin walking right away to prevent formation of restrictive scar tissue. Some papers mention that over aggressive manipulation of limbs after joint replacement could be involved in causing additional trauma to the limbs, that could initiate HO as it heals and forms scar tissue.

I do therapy involving the fascia called myofascial release. This is a gentle way to stretch fascia and allow it to remodel and you have to hold a gentle shearing pressure and wait. The fascia undergoes a change from a semi solid to a liquid like consistency and the facial fibers unwind and and become more mobile. If you are too aggressive in stretching fascia, it will tear and form scar tissue that just adds to the problem causing further restriction. With gentle facial stretching and patience, a patient can restore better body movement by removing tight restrictions that run through the body. My PT has worked on and stretched my surgical scars to help release this tightness. If you keep the fascia moving and sliding on itself, it keeps fluids circulating, and proper electrical conduction in the fascia itself as well as releasing any stored waste products. Some of the literature stated that HO was common among spinal cord injury and head trauma patients and the hypothesis was that HO can form when electrical signaling is lost to the tissues resulting from nerve damage. Restricted fascia is dehydrated and may no longer conduct electric signals. Some doctors embrace myofascial release therapy and others haven't heard of it. In medical school , they study dead tissues in cadavers that have lost all the properties of living fascia, so perhaps they don't think about it.

This video demonstrates how living fascia unwinds and reorganizes.
"Strolling Under the Skin" by Dr. Jean-Claude Guimberteau

So I wonder if myofascial release could help prevent HO? Some other considerations of course are how much movement is allowed post op after a spine procedure. I had a fusion with only a bone graft and no hardware, and had to stay immobile in a hard collar until the fusion process started which was 3 months. My PT was not allowed to do any MFR during that time, but after when I began rehab for weakened neck muscles because of immobilization, we also did MFR and it helped relieve pain by removing pressure. I had also done MFR before surgery so my tissues would be more pliable and easier to retract, making the surgeon's job a bit easier.

Here is our MFR discussion with lots of links to information:

Myofascial Release Therapy (MFR) for treating compression and pain: https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/

There is a provider search at http://mfrtherapists.com/

Here are links to some literature that I found about heterotopic ossification:

Medical Hypotheses
Volume 68, Issue 1, 2007, Pages 67-73
"The function of proprioceptors in bone organization: A possible explanation for neurogenic heterotopic ossification in patients with neurological damage"
https://www.sciencedirect.com/science/article/abs/pii/S0306987706004932
J Musculoskelet Neuronal Interact 2012; 12(4):230-240
Heterotopic ossification following traumatic brain injury
and spinal cord injury: insight into the etiology and
pathophysiology
https://ismni.org/jmni/pdf/50/06SAKELLARIOU.pdf
The Journal of Bone and Joint Surgery. British volumeVol. 90-B, No. 12
"Heterotopic ossification after the use of commercially available recombinant human bone morphogenetic proteins in four patients"
https://online.boneandjoint.org.uk/doi/full/10.1302/0301-620X.90B12.20975
So my question is this.... if you did myofascial release with a therapist to prepare yourself for spine surgery ahead of time, and also after surgery when appropriate, could you prevent the HO from happening? This may be something to discuss with your surgeon.