I can understand your reluctance about calling the farrier when your horse has foot handling issues. In all honesty, most of the farriers that I know carry a bottle of Rompum or Ace with them for horses that are unmanageable or dangerous to trim. From my understanding, it acts much like Valium, relaxes them and makes them not really care about what's going on without knocking them completely out. What I would do is go ahead and call the farrier and get it done by any means necessary but continue to work on his feet issues on your own time. He will certainly not learn anything by having them done when he is doped, but they need done and it won't hurt him.
As for having the trimming done while he is down for castration, I don't really know, that would be something to talk about with your individual farrier and vet. If they are both okay with it, I don't see a problem though the farrier may have a more difficult time judging angles and getting everything right when the horse is down instead of up.