The workflow that you describe here is indeed quite common. Currently however, git-flow does not provide any commands to assist you with this. Mostly, to keep things simple. Providing extra commands for this would increase the complexity of the tool's logic, which isn't a problem on itself, but git-flow is currently only a collection of shell scripts. And shell scripts aren't the perfect base language for extension and growing of tools.
I might include a feature like this in the Python version of git-flow that I'm currently cooking up slowly. Until then, please keep on using this workflow manually. You can simply use the plain old Git commands:
git checkout -b bug123 release/5
git checkout release/5
git merge bug123