Hi Ian
There are plenty of VSTi voices that can replace a Mega voice, (They came out long before Yamaha introduced their versions on the Tyros series) however the amount of work involved to make them match the Yamahas programmed styles is very labour intensive, (Just like the Tyros trying to control the VSTi Mega voice equivalents) it can be done, but it takes a hell of a lot of work.

Fortunately it is easy enough to swap the Mega voice track for a normal one, and then use (For instance) a VSTi rhythm voice. (It won’t sound the same, but the idea is to make it sound better, not emulate what is already there)

Bill