@Heavenfall
I tried the BuildingRequirement (ImprovementRequirement) type but it seems that it does not work for techs.
@Gnilbert
What ModType did you use to unlock the tech in your work-around?
I found another entry in the ImprovementType category:
RequiresCityImprovement + RequiredImprovement
But it seems that these have been replaced with the Prereq entries
and are, at least for techs, simply ignored by the game.
Thanks for your help until now, I will try some other things.
@seanw3
Here is the XML code I am working with at the moment: