Tim 'mithro' Ansell, there is a similar program for extracting capacitances of various structures. It is called FastCap. I believe the latest version of FastCap is called FastCap2 and is supported by a company named, FastFieldSolvers S.R.L. I don't know if FastCap2 is Open Source, you will have to check with FastFieldSolvers S.R.L. The web site for FastFieldSolvers S.R.L. is: https://www.fastfieldsolvers.com/

I have been using FasterCap as part of the Capiche project https://github.com/RTimothyEdwards/capiche. It is what I use to analyze the parasitic capacitances in sky130 and gf180mcu for the magic tech files. FasterCap is a revised/newer/better version of FastCap.