I want to set up a key shortcut so that the currently selected directory in dolphin will open up in deadbeef but I am having no luck in finding out how to do this. I am unable to find any detailed documentation for options to pass for dolphin or deadbeef.
Is there a way to do this?

No one has any idea?

I'm assuming you already have the file association set up such that deadbeef shows up in the Open With menu, and that you have another player that you prefer to use when opening the file via the usual options (click on the file, enter to open selected) (because if you only care about using deadbeef for music, then just setting that as the default for music files should be sufficient for your usage).

Examining available options (in the KDE 4 version of dolphin, I don't think this would have changed in the KDE 5 version), I don't see a way to add such a shortcut. The closest I could come up with include adding a Service Menu item for Open In Deadbeef which can be set up to show in the top-level context menu for music files (which only moves the option up a level in the menu), or examining how the Kompare toolbar button (plugin?) works to see if that can be easily adjusted to open files (from a keyboard shortcut?) in Deadbeef (probably requires some code changes).

Perhaps you might suggest this idea in the Brainstorm forum.