I've attached an Applescript that walks the dependency graph and figures out which actions are available. When the script completes, the available actions are selected, so you could use the script in combination with a filter that shows only selected tasks. If you run the script with a selection in the document, only the selected tasks will be examined, otherwise the entire document will be searched for available tasks. For a document of some size and complexity, this script will probably be a bit pokey, and I would suggest using a filter to select only tasks which are incomplete, selecting those tasks and running the script.
Known limitations:
I don't bother checking back more than one level of finish-start prerequisites if the first one I encounter is complete. If task C depends on task B being completed which depends on task A being completed, and task A is marked 80% complete but task B is marked 100% complete (it shouldn't even be started if task A isn't complete) I'll show task C as available for work.
If two tasks are connected by a start-start or start-finish dependency, the second task will not be shown as available until the completion percentage of the first task is > 0%. If work has started but hasn't been logged, I have no way of knowing that it is available.
I made a fairly complicated test plan, but no warranty of completeness and/or correctness is given. If you find a case where it seems like the script gets it wrong, whittle it down to a small test case or use the Help->Send Feedback command to make an anonymized version to upload here and I'll have a look.
See the
FAQ: Installing & running an OF applescript for instructions on installing and invoking the script. The FAQ discusses uses scripts with OmniFocus, but just read OmniPlan where it says OmniFocus and you should be fine.