My Goal
I would like the value of fixversion of a sub-task to always match its parent's value of fixversion. In other words, if the parent fixversion changes from v1 to v2, then all sub-task should also change fixversion from v1 to v2.
Why
The layout of the task board in Greenhopper filters items by fixversion, and I'd like all sub-tasks to be present under its parent. It's also true in our case that each parent item belongs to exactly one fixversion and that children of the parent should always belong to the same fixversion. I'd like to programatically enforce this rule.
(Side note: I'm a scrum guy by trade. I'd like to get my guys to fully adopt scrum and begin using Rapid Boards so I'm trying to gentlely nudge them in the right direction. For now, this fixversion deal is my incremental step in the right direction. But that's neither here nor there. 
Random Thoughts
Things I've tried or considered:
- Remove fixversion from the sub-task template. That's a no go. When doing this, there's has nothing to filter on, and the task board doesn't show the sub-task at all. When I add fixversion back to the field configuration, it shows shows again on the task board.
- Adjust the value when the workflow is advanced. This is a possible solution, but it's not ideal since the value doesn't get adjusted immediately. Instead, it only gets adjusted when a sub-task is advanced.
- Use a custom field to tightly couple parent to child. I don't believe you can configure the task board to filter off a field other than fixversion so unless someone knows different, I have to rule this solution out. Also, I've never tried to use a custom field in such a way so I'm uncertain if this would even be a viable solution.
Ideas anyone? I'm open to creative solutions. Hugs (and karma) to the person with the solution. 