I have, both in FF and other games. Other folks have talked about the various points of etiquette around this sort of RP. Effectively it's the same as any other conflict RP. There is an additional aspect of it that often doesn't show up in other RPs, though: Kidnapping RP takes place over time, so what do you do with your character between scenes? How does time flow?
Both in Eve Online and Star Trek Online I have played arcs that involved my character being kidnapped. In both of them we attempted to play it out real-time-ish. My character was taken on day X, and then was kept until freed. The arc in Eve went clumsily - I was new to scripted arc RP, and handled it poorly - but in both of them things got frustrating because I couldn't really RP my characters outside of the arc until they had been freed. The arc in STO was especially frustrating because we weren't able to get everyone together for scenes very often, so it really stretched on.
What that's led to in FF for me is the philosophy that you have to allow time to become decoupled for these sorts of arcs. Instead of pinning the start of the arc to the day it begins, I've started pinning the end of the arc to the day it ends. What that means is that if my character is kidnapped on the 1st of June and not freed until the 30th of July, I'm not keeping him out of commission for two months. Instead, we look at the IC length of the arc (say it's 4-5 scenes over 48 hours) and say the kidnapping happened on the 28th of July. It's pinned to the end of the arc.
This doesn't prevent consistency or continuity problems, of course, but neither does pinning an arc at the start. What it does do, however, is let the players take their time and play out the arc at their convenience without anyone having to put their characters out of commission ICly for an extended time.
Both in Eve Online and Star Trek Online I have played arcs that involved my character being kidnapped. In both of them we attempted to play it out real-time-ish. My character was taken on day X, and then was kept until freed. The arc in Eve went clumsily - I was new to scripted arc RP, and handled it poorly - but in both of them things got frustrating because I couldn't really RP my characters outside of the arc until they had been freed. The arc in STO was especially frustrating because we weren't able to get everyone together for scenes very often, so it really stretched on.
What that's led to in FF for me is the philosophy that you have to allow time to become decoupled for these sorts of arcs. Instead of pinning the start of the arc to the day it begins, I've started pinning the end of the arc to the day it ends. What that means is that if my character is kidnapped on the 1st of June and not freed until the 30th of July, I'm not keeping him out of commission for two months. Instead, we look at the IC length of the arc (say it's 4-5 scenes over 48 hours) and say the kidnapping happened on the 28th of July. It's pinned to the end of the arc.
This doesn't prevent consistency or continuity problems, of course, but neither does pinning an arc at the start. What it does do, however, is let the players take their time and play out the arc at their convenience without anyone having to put their characters out of commission ICly for an extended time.