I see, so the mouse button is the "modifier". Sure, that's a good idea, but as you say, it's not a single key trigger in the real sense anymore. 
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I hate to contradict the master. Actually, I don't contradict him at all. I only add to his knowledge.
In the big beautiful world, there is not only KM. Other apps can recognize text fields and act accordingly. 
Well...
Apps can do this with their own inbuilt shortcuts. Final Cut Pro can accept "v" to make an item in the timeline enabled or disabled and happily accept "v" in a text field as the letter v.
But that is because the code is written at some deep level in the App. As far as I know other Apps (including Keyboard Maestro) have no access to that level of coding when they are interacting with other Apps.
Ah, I can't quite keep up technically. But I meant it differently.
Other apps show you the name of a field of any other app where the cursor is. And once you know the name of a field, you can tell that app, if the cursor is in this field, then do this, not that ... (not with their own inbuilt shortcuts) hope that makes sense. 