This is just for informational sake as Alt/Option-Double-Click is a perfectly valid method and for most I would assume much less effort than what I am about to list.
I may be misunderstanding your desire for a keyboard shortcut, but I can essentially do the same as you describe by the following keyboard commands. Third-party utilities could offer you a way to make these into a single keyboard shortcut for your purposes.
- Start with a selected task or select a task* (I presume we are speaking about being in Context mode)
- Use 1st keyboard shortcut Command-Control-F to get focus on the project of selected task
- Use 2nd keyboard shortcut Command-1 to switch to Project Mode and see all related tasks in that view
* To select a task via the keyboard:
- Use keyboard shortcut Command-F to bring up find
- Type the phrase of your task
- Click Enter/Return to select the task in editing mode
- Click ESC to exit editing mode
- Proceed to step two above
Just use Spotlight and type in the name of your project instead of doing everything I list in this post. When a project is selected in spotlight it opens in a new window showing all of the related tasks.
- Command-Space opens Spotlight (or whatever keyboard shortcut you have assigned to this function)
- Up/Down arrows select Spotlight results
- After selecting the project, Enter/Return opens new OmniFocus window with the project selected
The caveat with using Spotlight is two-fold;
- The result, as stated above, is only a selected project without focus on that project.
- When you use it without OmniFocus running and you have multiple versions of OmniFocus on you Mac (e.g. OmniFocus 2 Beta). In this case Spotlight will open in the "OmniFocus" application; this means by default OmniFocus 2 Beta will open instead of OmniFocus 1 if neither application is running. The reason for this is when installing the OmniFocus Beta it renames the current OmniFocus 1 application to OmniFocus + "its version number" — for me this is OmniFocus 1.10.4 — and Spotlight opens OmniFocus documents in an application named OmniFocus.
Hope this helps.