Now, back to the original programming.
If you cannot treat this horse's hind legs, get a heavy duty portable panel. Put it in a safe fence corner. Attach it to one side of the fence about 1 foot from the corner. If you wire it up, be sure to wire it securely in 3 or 4 places. Put the horse's head in that corner and bring the panel around to its hip and you have an instant set of stocks. You can doctor about anything in a 10 minute set of stocks like this. You can fasten a rope around behind the horse so it can't back up.
I would be very careful about washing off the wounds below the hock. Water is REALLY BAD at encouraging the over-growth of proud flesh. Proud flesh needs to fill n any gaps in the skin and then after that, you do not want any more of it.
One good thing about Underwoods is that you do not have to worry about infection if you use it. It evidently kills bacteria or is at least bacteriostatic.