Well I've been having some fun playing with this idea. First of all, the answer to my main question is "yes"... it is trivially easy to remove a pack of FP100-C from a camera, do something with it, then put it back in and pull the tab.
You don't need to worry... it will work the first time and it is easy to do it in the dark. Far easier than handling 4x5 film for example.
I made a pinhole copy of my paper negative onto FP100-C, and it looks neat, it has some blue cast, but looks nice.
I used the same instructions bvy linked above to "reclaim" the negative and it too was easy and worked very well. There is an obvious positive image on the negative. I don't use color film very much, but it seems a bit low contrast and maybe thin. My homemade 122 carrier was actually too big, but it fits in a 6x9 carrier very well.
So... the only problem I've run into is a mundane one. With the bellows fully extended on my enlarger, I can't make the projected image small enough. I think there is some way to use an enlarger to make reductions or 1:1 copies, but I don't know how to do it, so off to read about that...
This is kind of funny:
I made two test exposures of a part of the image. It was difficult to see well enough to focus. The first one I stopped down a couple clicks and counted out about 20s. It was obviously underexposed ( too light ). So the 2nd one I opened the lens all the way and let it go 15 seconds. Whoa! even lighter! OOPS! I'm trying to make a POSITIVE! Lighter = overexposed! I'll try another one soon with a shorter exposure!
Edit: the "reduction" was easy: switch back to my 50mm lens and stick a book under the easel.. then extend enlarger bellows and find the place where focus is achieved.... a little back and forth to get it right, I think I actually ended up making a slight enlargement. This time it was easier to see to focus. I was tempted to push the negative into the FP100-C pack and try for a contact print.... results soon but I have to get back to work right now.