Wheatfield, I think you have to try the K-x to see what I mean
Based on what I've learned this is how it works:
As you said, trap AF only works when the AF selection is set to AF.A or AF.S, not AF.C
On a body with a dedicated switch to select these modes, such as the K-5, you switch the nob to AF.S and you are good to go to use trap AF.
On the K-x, we don't get that luxury. The switch on the side only switches between AF and MF. Choosing between AF.S and AF.C is done within a menu.
Situation: Let's say I was using an auto-focus lens, and I wanted to use AF.C. I set it to AF.C and shoot, which works great. Later, I switch the lens to to a manual-focus lens, and I enable trap AF. As you've mentioned, trap AF does not work with AF.C.
Heres the caveat: As we have discussed, trap AF only works with AF.S. So, if I attach a manual focus lens, I need to change the mode from AF.C to AF.S. The "bug" I mentioned is that the menu to change from AF.C to AF.S becomes inaccessible if a manual focus lens is attached. The only way to access this menu is to swap on a auto-focus lens, which re-enables the menu, then swap back on the manual lens and shoot happily with trap AF.
This isn't a problem if I remember that I reset my mode to AF.S, but let's say one day I had AF.C on with an auto-focus lens and shoot. The next day, I only take a manual focus lens with me. I realise that I forgot to change the mode back to AF.S because the menu is greyed out. Now I have to trust my poor eyes to focus correctly, and I have no way to fix this until I head home, slap on a auto-focus lens, and access the menu.
I hope this clarifies things, because I think we're on different pages. Cheers!