More of a long-term wish. This came up again and I finally found a library that appears usable for our purpose: https://github.com/stepcode/stepcode. Its license is 3-clause BSD and the build system is CMake, which already solves many integration troubles.

The main issue here is representing arbitrary non-parametric solids. Since the STEP files have identifiers assigned to every entity, we could deterministically derive our identifiers from those; but beyond that it's unclear to me how such entities would integrate into the existing system. @jwesthues, any advice?