In message Pine.LNX.4.21.0007061001460.13752-200000@balloon.cs.uni-magdeburg.de you wrote:
Now, the recent external primitive rework has broken this. To look up an internal function, the program module itself must be loaded via dlopen(NULL,...) - something that has vanished from the sources.
If I'm understanding your problem right, this is done by adding your function to the list in the file platform.exports which then adds it to the list of internal prims.
tim