I would think it's a good idea as well - plenty of kids just do the high scaling subjects (i.e. 4U maths, physics, chem), and then pick a course based on what ATAR they got, which often or not tends to be combined law. And then they realize they make a wrong choice because in the end they don't like it due to the large amount of readings, as well as class participation involved.
Also I believe that with the current system, I'd definitely can say that you in order to get the ATAR required for law, you have to work much much harder with humanities subjects compared to those who do 4U Maths, for example. I know plenty of students who missed out ATAR wise, even though they were clearly capable of doing law, but unfortunately they missed out even though they got band 6s. Having a test like this might be able to discriminate those who are actually good at doing what is required for law, as opposed to those who just rote-learned their way through the syllabus. It's also an opportunity for those who were pretty close off but didn't make it into the law, to give them a chance to gain entry.
I for one though, would ask unis to reduce the number of law places out there - there's too many law graduates out there and not enough jobs. As they say, law is the new arts.