The plugin available at http://confluence.atlassian.com/display/JIRACOM/Available+Worklflow+Actions+Customfield should be included in jira. However instead of making a custom field directly a field that ONLY exists for issue navigator should be considered.

From email:
One of the nice things about sub tasks is that the workflow actions are
in the task in an easy to use manor. Can this type of display be added to
the navigator for all issue types? I know that a plug-in was written for
that and posted to confluence, but it has a few quirks and it would be nice
to have an official implementation.