I'm thinking that if it were implemented, it'd have to be yet another configuration option to toggle how keyboards were handled-- which would then lead to people turning it on blindly and asking all sorts of support questions about why other keyboard-sending joystick devices or keyboards were no longer working as they should.
That reason alone makes me think it should be handled outside MAME or as perhaps a platform-specific option as compared to core.
That assuming that anyone wants to code it. I'll be waaaaay over here not writing code for that puppy, myself. I don't even have one to test with, anyway.