Possible to have separate shortcuts for different text-to-speech voices?
While I couldn't find a way to assign keyboard shortcut directly, an acceptable solution turned out to be much simpler than one might think.

Create a new Service in Automator. By default it takes selected text in any Application and lets you to do something to it. Choose Speak Text from the Text category and select any active voice from the drop-down. Now save the new Service under some descriptive name and you will be able to assign a desired keyboard shortcut to it from: Preferences > Keyboard > Shortcuts > Services > Text > "Name of the Service".

Done.