They could simply provide API with black-box interface or even run C/C++ user defined interface/functions similarly how PostgreSQL handled it:
https://www.postgresql.org/docs/current/static/xfunc-c.htmlFor an example, I've enable PostgreSQL Query to run C# functions with the use of Inter-process Communication Protocol with the API above which is demonstrated in this proof of concept:
https://github.com/CsharpOnLinuxDev/IPCPostgreSQLPoC/tree/master/IPCPostgreSQLSo everything above can still satisfy their requirement to keep some of their internal code secret while allowing us to expand game engine capability.
Obviously expanding the game capability this way would raise question about the security concern, so it would be necessary to open source mod codes so that it could be audited, but regardless, it's a huge plus to have.