Good question! I'm following up on this for you. I found a post from 2018 that says you cannot add ISP to a flash briefing but since it's 2019 now I'll see if that's changed.
As for the eligibility, the only types broadly ineligible are Blueprint skills https://blueprints.amazon.com/ There are other eligibility requirements of course, but Blueprints are the only explicitly excluded format. I'll let you know what I find out about ISP in briefings though.
I found out that only custom skills support ISP and flash briefings are NOT custom skills so it would not allow for in-skill purchasing. I imagine you can create a custom skill with similar functionality, though.