Yep. Korgs have been able to do this for years....actually does a bit more than what you listed too!!

I do believe Scott Yee has been asking Yamaha US for several years now to simply be able to link a registration to a MF entry...

It does not require a great deal of programming (on Yamahas part) either..Quite a small amount of code is required.

The only thing I can surmise, is that Yamaha are CHOOSING not to...and I guess when you look at the millions of folks using Yamaha arrangers, the number of those asking for this link is like one grain of sand on a beach....