I recommend putting the jacket into some kind of bag then strapping it to the top of your trunk bag.
Too many times I've seen the sleeves work their way loose and flap around. You do not want them getting into the spokes.
A bag will also keep the jacket cleaner and drier. A small plastic shopping bag is fine.

Another approach is an ultralight drawstring backpack. They work great for bulky but light and soft things like jackets, baselayers, and tights. Good if temperatures are below 75F.