So, between two crashes,
I tried to add this in the "Stats section" for an improvement:
<!-- Stats -->
<Stats>
<EffectType>Level</EffectType>
<Scope>Global</Scope>
<Target>
<TargetType>Improvement</TargetType>
<ImprovementType>Manufacturing</ImprovementType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Stats>
This works at a global level, each manufacturing improvement built on any planet adds 1 level to all manufacturing improvements built on all planets.
So, this can be used to make a specific improvement, per example limited to 1 improvement per civ, as it's quite powerful.
I tried next to limit the effect to only the improvements built on the planet, with this:
<Scope>Local</Scope> --> Allowed, but does nothing.
<Scope>Colony</Scope> --> Not allowed.
<Scope>Planet</Scope> --> Not allowed.
Is that possible in the current game engine?