Add a new reason that exceptions could be granted. When the app needs a newer version of the library and the library upstream has the necessary changes in their tree; they just haven't made a release yet. The following section would be added to [Packaging:No_Bundled_Libraries#Some_reasons_you_might_be_granted_an_exception]
Needing unreleased features
When an application needs unreleased features of a library and that library has committed to those features (usually, the changes are checked into the trunk branch of the upstream's revision control system) but the library has not yet made a release that has that code an exception may be granted to bundle that library until the Fedora packages contain the necessary extra features. Note that for this we would definitely want the standard questions regarding timeline for the change to be merged and unbundling to occur to be answered so that we can make sure this is fixed should the package maintainer disappear or get busy with other things.
As noted earlier, bugfixes are rarely a good reason for this. Most of the time bugfixes should be backported to the current Fedora package rather than bundling a library.