Actually not much different. The MS GUI framework that SketchUp uses, allows this but the implementation is bugged.
To see the bug, first put the toolbars into editmode by opening the View > Toolbars… edit dialog.
Once the toolbars are in editmode, each toolbar button has a context edit menu available (which is not accessible when in normal application mode.)
Choosing the “Button Appearance…” item on the context menu will bring up a dialog to customize the button.
(Pardon my dark charcoal theme)
If you choose to display “Image and Text” and if the text was not hardset to the command name, you could edit it to make it the shortcut.
In my example, I was editing the Orbit button, and attempted to set the button text to “O” but SketchUp keeps resetting it to “Orbit”.
So either a bug report or a Feature Request is in order.
ADD: I think that there has already been a FR to add the shortcuts to the tooltip popups.