We've decided that to track which case belongs to which (released) version we are going to use the "Version" field in FogBugz.
The workflow is as following: Start each case with a blank version, then when we've decided to release a new version of our product, we update all closed cases with blank version field to the newly released version.
However, upon trying to do so in FogBugz we've stumbled upon an issue: "Version" field is read-only once the case is closed.
Options and alternatives:
- Version field - We can use the version field, if we either leave the case at resolved or reactivate the cases before updating the version field. Both options are bad in my eyes.
- Custom fields - I've figured I could create a custom field that would be editable when the case is closed (note: when the case is CLOSING, the field is available. However, when it is closed it isn't). However, the custom field seems to read-only when the case is closed and there is no option to make it available.
- Release Notes - This seems like a relatively good choice since "Release notes" is available for closed cases and we don't use it for anything else. However, "Release notes" aren't visible in the grid. Moreover, I can't update release notes when I am editing several cases at once.
- Tags - This is currently the only option I see. Tags are available to closed cases, visible in grid and can be multi-edited. However, I'd prefer a more specialized field for this (e.g Version), especially since we do use tags occasionally for other stuff.
Custom fields clarification
To clarify the issue with custom fields, here is a screenshot of creating a custom field:

And here is what it looks like when I try to edit a closed case, note that I have no ability to edit TestField:

I'd be happy to hear of other alternatives and options.
Thanks!