I searched for a while trying to find a wagon harness for the Hivewire Horse, but the only tack I can find for him is in the Marketplace here, and nobody did any harness tack, just riding. Or at least, I can't find any that anyone did for it. Before I put time into hunting through all the Mil Horse tack, has anyone successfully used Mil Horse tack on Harry without ripping most of their hair out during the process?

There's a Tack Conversion Kit for Hivewire Horse made by Lyrra Madril that convert Tacks for Millennium Horse convert to fit Hivewire Horse, Unfortunately hivewire3d since closed down and haven't put it in Renderosity. Maybe someday Lyrra put and update it in Renderosity (like Daz Horse 3 Tacks convert to Hivewire Horse only DS)

https://community.hivewire3d.com/threads/lyrras-products-at-hivewire.1593/

That link connects to a 10-year-old post. If it hasn't made it to the Rendo marketplace by now, I doubt it will ever appear. My suggestion is to contact the vendor directly; however, I haven't seen her active here for years.

It may be a job for the morph brush. If I can fit a saddle on this guy, I'm sure you can do it.