Funny you mention that, I was just in Singapore for a conference, and wearing my green hopsack jacket. Outdoors it can be a challenging, I walked nearly 1km from my hotel to the MBS convention center in trousers, a shirt and the jacket. And I admit, I was within 200m away, at which point I gave up and had to the jacket off if I wanted to show up the conference dry, ha! However, when indoors, it's perfect for a jacket IMO. AC is always blasted and the temperature indoors can actually be chilly and call for a jacket.
I think a breathable wool hopsack jacket like this would go great with something formal like a high twist wool trouser and on the other end of the spectrum you could even wear casual cotton chinos or even jeans.
You could wear it casually with jeans and a shirt--but this i'd only recommend with a waistcoat fabric that is also casual, such as a tweed. If the waistcoat is from a more formal suit fabric, it's harder to break it up. However, you could still just wear the suit trousers, a shirt and the matching waistcoat for a more casual laid back look. This also works in warmer weather where a jacket might be too warm.
