The controller_timelog_edit_before_save hook is available in several of the TimelogController's methods. However, since TimeEntries can be created when updating an Issue, I found myself needing the same hook in IssuesController.new().
There is no hook in IssuesController.new() that lets plugin developers work with the TimeEntry.
I've attached the proposed hook-addition as one patch, and additionally a patch containing the plugin that utilizes it as an example.
I realize that it might be bad practice to have a hook named 'controller_timelog_*' outside of the TimelogController, but I think it makes sense to be able to use the same hook, as the plugin demonstrates.