I like the idea. People with less holdings aren't held in prison for very long. It makes sense and would be a good gameplay addition.
As far as the long prison times originally suggested, I'd rather Duke/Council members have a sure shot of losing their position unless they pay the ransom, Lords could get a random chance of being released in time or losing position. The ransom should also be higher for higher positions. You want to enable people to join the fight if they want to without risking weeks of downtime, but still risking their prestigious positions and/or chests of gold.
In modern prisons, prisoners spend their time exercising and getting degrees (at least according to films). Maybe imprisonment wouldn't be such a drag if there was an opportunity for prisoners to increase their skills at a faster rate than they normally could. So every hour they spend studying or training in prison is like 2 or 3 hours on the outside. By the time they get out they could have mad swordfighting skills, and it's less boring for the player.
Might not be a bad idea. Maybe some of the softer skills would be good. Oratory and Trade (sweet talking guards, trading a few gold for smokes/wine).
