I have a plugin for IntelliJ's Webstorm/Phpstorm that does just this and it's amazing - whenever I use an item that has a shortcut it pops open that short cut so I can slowly learn them.
If I also use something that doesn't have a shortcut 3 times within the programs running then it asks me if I want to set a shortcut for that action.
I think as time goes on Google will eventually be on this route considering how well it works (at least with the first portion of showing the shortcut command whenever I use it).
However that still wouldn't solve the problem of the users having to learn the commands from scratch first so if they don't know it exists they would never learn the shortcut for it in "OK Google"
Using the search predictions and history as cues, if you're doing something that has a voice command would work too. Like if you start typing out "weather near" have a tooltip of the voice button with a caption like "Try with voice?" that you can tap and it would then turn on the mic and give you a karaoke style prompt with blanks for you to fill.
If I also use something that doesn't have a shortcut 3 times within the programs running then it asks me if I want to set a shortcut for that action.
I think as time goes on Google will eventually be on this route considering how well it works (at least with the first portion of showing the shortcut command whenever I use it).
However that still wouldn't solve the problem of the users having to learn the commands from scratch first so if they don't know it exists they would never learn the shortcut for it in "OK Google"