Hey Andrew,
An interesting idea but I don't know whether I want uncomfortable looking sleeve inserts either.
One thing that I have found 2 b effective is the LDComfort riding t-shirt with long sleeves. The instructions say 2 wet the sleeves and any breeze that gets up the arms of your jacket creates an evaporative cooling effect. This garment also takes the perspiration off the upper-body and creates the cooling effect that way as well. Just needs a rinse and hang at the end of the day to refresh for the next day's riding.
I tried this out last summer when I had a leather jacket only and it worked OK, but now I have a summer textile jacket with removable lining which I will b using on the way 2 Ballarat. I reckon it will b gr8 in really hot conditions.
Now 2 solve the hot seat problem!