Domain Expert is a person who has knowledge of a particular system. With such knowledge it may seem easy to design APIs for the domain. However without understanding the API Paradoxes the quality of such API may not be high. It is likely going to cover the domain field, but the API usability or readiness for evolution will very likely suffer (unless such Domain Expert reads TheAPIBook first).
This is a new situation for me: In case of NetBeans or in case of HTML/Java APIs, I was also the architect of the system. I knew it by heart. Now I barely understand how Truffle works and what makes it the fastest execution system for dynamic languages. My biggest fear is that I will design something that will be inherently slow.
On the other hand, I am not yet damaged with the expert knowledge. I can still see the system with new comer eyes - just like you, users of Truffle will. As such I can perform a usability study on me, at least initially.
If I can design easy to use APIs for Truffle, then I can create a perfect API facade around everything! Soon we'll have a chance to see whether one can be good API designer without being real Domain Expert. Soon we'll find out if API Design can be offered as a service!