I don't know if there are specific key commands to do this, but there is a workaround using Full Keyboard Access:
1. Under the view menu, select
Customise Toolbar... and add the
Sections button to the left side of the toolbar (if you are going to be doing this very frequently, you probably want to make it the leftmost button).
2. Ensure FKA is on in
System Preferences>Keyboard and Mouse.
3. When in OmniOutliner and you want to hoist a particular section, press
control-F5 to activate the toolbar buttons,
tab to the Sections button (note, you can skip the tabbing if you made it the leftmost button, as it will already be selected). Press
space to click the highlighted button and open the menu. Use the
up and down arrows to navigate to the section you want to hoist. Press
space to select the highlighted section.
You can watch a quick screencast of this
here - in the latter half, after moving the section button to the left, I am using the keyboard to access the menu.