Wouldn't it be helpful to have context sensitive shortcuts available via right-click?
For example, when in the crop tool, a right-click would include 'Shortcuts' in the sub-menu. On selecting 'Shortcuts', a pop-up displaying all available shortcuts, including 'X' to toggle the aspect, could be displayed.
Dave feature requests go on the Photoshop.com site. Please do place yours there where it will get the attention of the team. Of course new features for Lr4 have finished as the bus has left already.