In practice, you can effectively do this by assigning an applescript to a key-stroke using something like Keyboard Maestro or Fastscripts.
The following, for example, makes a shared duplicate of the bottom layer of the selected canvas. (And once you were happy with how it worked, you might choose to uncomment the
delete line, to discard the original non-shared copy ...)
Code:
tell application id "OGfl"
tell canvas of front window
set oLayer to last layer
set oShared to make new shared layer at end of layers
duplicate graphics of oLayer to end of graphics of oShared
-- delete oLayer
end tell
end tell