I don't think I want to add to many specialized shortcuts, but if you ask in the hogbaysoftware support forums I'm happy to write an AppleScript to do this, and then that can be assigned a keyboard shortcut.
Also generally for collapsing items I use outline mode: Press escape key, and then you can just use left/right arrows to expand collapse items. Maybe not what you are after, but that's how I find I'm always collapsing/expanding things in Bike.
In OmniOutliner, the collapse row shortcut will collapse the parent which takes care of the specialized extra keybind. What seems odd is OmniOutliner and Bike's outline/collapse row keys are reversed with cmd-0 and cmd-9 doing collapsing and expanding in one and expanding and collapsing in the other.
Also generally for collapsing items I use outline mode: Press escape key, and then you can just use left/right arrows to expand collapse items. Maybe not what you are after, but that's how I find I'm always collapsing/expanding things in Bike.