I think its more a problem of universities not being more stringent in requiring students to take specific courses related to the discipline they want to study in uni. However, whilst I do understand that its far from an ideal metric for engineering, I still think that the current system is fairer for a wider spectrum of students than it would be if entrance was based on something like the SATS.
To my mind, intelligence is too ambiguous a concept to accurately measure with any generalised type of test. Plus, I think this type of system would only serve to exacerbate the already prevalent issue of people seeing school as something they need to be 'coached' for rather than for the simple (yet important) purpose of getting an education. In excelling at these 'creative/ artsy' subjects, the student who got high marks in art and music displays talent, and more importantly, work ethic and creativity. Even if they're somewhat talented, there wouldn't be too many people that could get 95 in VA and music without putting in significant effort to those subjects. And whilst of course the science/ maths student could be very well be hardworking/ intelligent/ talented also, it would be totally unfair to diminish the achievements of the person taking creative type subjects because they used their intelligence in a different way.
This is obviously just personal opinion, but if I had to define how 'intelligent' someone was i'd see a person's ability to think creatively as a hugely significant aspect. Whilst i'm not trying to deny the benefits of being able to think logically/ analytically, often the people who we see (in any field) as being the most successful are those who can bring a creative/ fresh approach to the problems of their discipline. And it's for this reason that I think people's performance in creative type subjects should be valued no matter what course/ career they are looking at, and thus we shouldn't change the current system. If anything, I think that the current system should be adjusted to recognise the achievements of people in these type of subjects more.
As long as the VA/ Music person took the recommended maths/ physics courses for engineering, I can't see why their their type 'intelligence' is less relevant to their course than the other person. Also, (which is probably something I regret in my own subject choices) I think it's important to encourage people to be as diverse as possible with their HSC courses, which I think a standardised test would discourage. Just my two cents