I think the only way you would have trouble cashing out is if you're supporting a minor faction and they had no control over any stations anywhere else. It's a big galaxy so I guess that would be a problem somewhere. So would you have to go to an interstellar factors to cash out? Might be a good work around.
If you're happy to just take the money and not care about the result, sure, it's not a problem. You can
- cash the bonds in a different system where the minor faction is present
- cash them at IF
- wait for the war to be over, wait for rep decay to bring you back to unfriendly, cash them in
If you want the bonds to actually affect the result, you need to cash them in the system with the war, while the war is going on. If the faction you're fighting against controls all the dockables in the system (and it's a big galaxy, there are plenty of systems which only have one dockable in the first place) you can't do that when you fall to hostile.
Fair, realistic or otherwise ... it gives a massive defensive bonus to certain factions.
The way I can see to work around it is:
- take a bunch of data delivery missions for the faction you want to fight *against* to stations they don't control
- fight in the CZ
- if you get to hostile, hand in delivery missions (taking the 'extra rep' reward where available) to return to unfriendly. As there's a war on, this won't help the faction's influence.
- hand in the bonds
- pick up some replacement delivery missions, repeat.