Whilst my personal preference is not to exchange these historical characters, it would be really nice if they could go "back into the pool" for possible addition to another FM later (assuming the character is still alive.) Maybe there could be even be some mechanic (not Charles Bronson) which determines whether they survive which is based on the circumstances of the death of their FM.
This is already possible. I took a look at how ancillaries work, and if they are "Unique", they can only exist once at a time. We can also make ancillaries appear between certain years, and if one character would die, assuming that we're still in the right time period, an ancillary can reappear.