Could Agave syrup be used as a substitute for Golden Syrup?
It'd probably work. It won't be quite the same, as golden syrup has a distinct flavour of its own, but it may well still be quite tasty.

Agave syrup, as far as I recall from last time I used some, is a bit runnier at room temperature so depending on the recipe you may have problems with it being too liquid unless there's something else holding it together.

Flavour-wise, I believe the golden part of golden syrup comes because it has a small amount of molasses in it, so you might try adding a very small amount of that - but it may not be a strong enough flavour to make it worthwhile to go to that much trouble. As you mention, agave syrup has its own flavour so the result could still taste great.

The big thing to be wary of is comparative sweetness. Agave syrup is almost entirely fructose, while golden syrup is a mixture of fructose and glucose. Fructose tastes sweeter, so agave syrup may oversweeten if you use it 1:1 as a substitute.