Yes, they have considered it.
A few years ago (gosh, maybe it's been like what... 5 years now, or longer?), they had some Timberliners and were testing them on Voyage, but for whatever reason they stopped and it never happened. I can't remember why it never happened, but they have most definitely considered it.
EDIT: After some research, it turns out they were testing them in August 2010.
According to Coaster-net.com, the Timberliners were scrapped because they ran slower due to their lighter weight. They claim Holiday World highly values the Voyage's intensity and the slower running trains took a lot of the bite out of the ride.