You may need to put in a bolt behind the seat. We recently had to do this in a work ute, its a duel cab but didn't have anywhere for the seat to attach. We also had to do this in our Holden Rodeo. A few of my friends have an Ftruck & have their carseats in there, I presume they would have done the same.

If you have the seat attached correctly etc then I cant see why not. No everyone has a car, so effectivly you have to be able to transport a child around in a ute if need be so I cant see the law as saying "no", but that's just my opinion. How many days a week? If it's not all the time, you are happy with it & seat is attached properly then I say "why not".