Step !. - identify what species of tree the parent tree is. In the process, measure the length of the normal tree's needles. This will figure into whether the species is worth messing around with for bonsai purposes. Generally, if needles are longer than 4 inches, it is not really worth the effort to turn it into bonsai.
Step 2, - get some needles from the witches broom and measure their length. If the needles are not less than the "normal tree" and or not less than 4 inches, it is really not that good for bonsai.
Step 3 - if the needles of the broom are less than 4 inches, it might very well be good for bonsai. Next step would be harvesting scions for grafting and or doing an air layer. Note: I would assume air layer has a very low probability of working. But you never know. Get permission. Climb in tree to the witches broom. Or if that does not work. Common redneck way to harvest witches broom is a shotgun blast to the supporting branch. Do this in middle to late winter. The whole broom can be knocked out of the tree in one blast from a 4-10. The scions harvested can be wrapped in moist sphagnum and stored in the refrigerator until ideal time for grafting in late winter, early spring. Ideal is understock has just begun growing for spring, and scion wood is still dormant. Choose your understock by what pines grow well in your area. Scots pine, P. sylvestris is one of the "universal understocks" but others could be used, JBP, P. nigra, P. taeda, what ever you want. You can use seedlings of the normal form of the species since you identified it in step 1. Make at least 10 to 20 grafted trees with the witches broom. This way you can evaluate them more effectively. And you have some to spread around should you choose to share.
Step 4. observe the growth habit of the your witches brooms. Come up with a description of how the broom compares and differs from the normal form of the species.
Step 5 - if your broom turns out to be really different, stable as a broom long term, and worthy of mass production, now it is time to name it, and register it as a new variety. The American Conifer Society can help you with the registration. THen you can patent it and trademark it.