I do not know which of your options would be coolest.

I live in South China, just above the tropics, so the weather here is pretty hot and steamy most of the year (4 seasons: summer, December, January, February). It's pretty much like my hometown of Orlando.

One of the upsides of China is that custom tailoring is relatively affordable. So I have a couple of kilt jackets made with the local climate in mind. One is a tweed number made for me by Yuen Tailors in Hong Kong, who actually specialize in Highland wear. Although it is a beautiful dark green tween with blue and orange overcheck, it is a relatively lightweight material--much lighter than my old tweed sport coats from the USA, or my Harris Tweed casual jacket. So, in warm but not brutal weather, I can wear that jacket, often without the waistcoat when things are humid but still "late spring" temperatures.

I also have a kelly green kilt jacket my Beijing tailor made for me, using a cotton shirt fabric. Although it gets a little wrinkled by late afternoon, it works for those hot weather occasions when I need to be dressed up (awards ceremonies at our school, for instance). It is barely heavier than a second shirt, and very breathable.

So you should probably try one of your options and see how it feels. It may be great, or it may be only useful for the winter months. But then you can use it as a pattern when you do have a proper linen or cotton jacket converted for kilt use.

Good luck!
Andrew