Hi Zuki
I was talking to the Roland Rep at one of the UK Festivals and apparently the Scat voices on Roland’s are actually copyrighted.
Regarding Yamaha, try some of the dedicated forums as various third party sounds are available. (Don’t know if there are any Scat voices though)
BTW the first Scat voices on a keyboard with accompaniment were actually introduced in 1989 with the Wersi Live style sound sample system that could be fitted on their CD line instruments.

Bill