It wouldn't be an issue for him to be involved, he was finishing up DBZ manga while working on CT as well as the fact he was working on both Dragon Ball manga, character designs for Dragon Quest and a tie-in Dragon Quest manga all around the same time in the late 80s early 90s. So I feel he wouldn't have much issue doing both, especially since CT would be a relatively short term project as opposed to Dragon Ball.
Akira Toriyama did have a bit to do with the game, CT came about after Sakaguchi, Toriyama, and Horii all met each other at an electronics expo and started talking shop, which eventually led to them deciding to make a game together. Toriyama wasn't just the character designer, he was also the world designer and set a tone to the game which is why it feels a bit more light-hearted and whimsical despite some of the dark subject matter CT had going for it.