KC,
Thanks for all your work.
I maintain OpenVSP (
www.openvsp.org) -- a parametric geometry tool that originated at NASA, but 'left the ranch' when it was released as open source in 2012. It is still heavily used (and development is supported) by NASA, but releases are independent.
OpenVSP uses STEPCode to write AP203 files that currently contain only B_SPLINE_SURFACE_WITH_KNOTS entities (and the boilerplate to support them). Someday we may write full BREP's, but that isn't in the works. OpenVSP does no STEP import whatsoever.
STEPCode allows us to support workflows that start in OpenVSP and then transition to CAD/CAM/CFD/FEA/etc. tools.
We have similar support (added much more recently) for IGES that relies on
http://cbernardo.github.io/libIGES/ . In both cases, supporting these formats would have been impractical without a high quality library to get us started.
I also contributed the ap203min example to the STEPCode source to serve as a starting point for others who follow down this path -- it is very similar to what appears in OpenVSP.
Rob