All, having been to the wedding now, I'm replying to my own post, for the benefit anyone else is in the same position in the future...
- Smart casual is appropriate. Most people were dressed nicely, but not over-the-top. I was probably the only person wearing a tie

, but it's better to be too smart than not smart enough.
- Everyone comes and goes pretty freely. The event went on about 5 hours, but as long as you stay long enough to give your best wishes to the couple, that's fine.
- Give your gift (money is fine, in either green packet or just an envelope inside a nice card) at the end, as you leave. Compare this with Chinese weddings where you give the gift as you arrive.
- Other Malay/Muslim courtesies, as usual.
Good luck.