I finally figured it out - I remember this from a What's New 2016 and remember it being really tricky and stupid but I dug through some old youtube videos and re-discovered it.
To attach a bolloon or note to custom properties of a subassembly:
- Create the balloon or note, attach it to whatever component of the subassembly you are trying to grab.
- Select link to property Choose selected component or component annotation is attached to etc.
- Right click on the drawing on the thing you are trying to select
- Choose select other
- Choose the sub-assembly from the pop-up
- This can be very un-intuitive because the pop-up doesn't actually have the subassembly name. It's kinda weird how they are named in the pop. Pay attention to your feature manager tree, from there it will highlight the subassembly and you will know you've picked the right one.
- Click OK a bunch.
- Drink beer and enjoy your hard work. Receive infinite praises and raises from your boss. Take the stock option and revile as your companies profits rise. Retire to Hawaii with Trump money.
Again, I've made an enhancement request for this, please search and vote for my enhancement request: