Special Rules can only benefit from "Entwined Power" and similar effects if the Special Rule modifies the process of choosing which land/Spirit you're targeting with a Power (e.g., for meeting a Sacred Site / terrain targeting requirement or moving Presence as part of targeting) and only as part of that targeting process.
Since "Slave Labor" only triggers in lands that had 2+ Explorers before Building, the Explorers built via "No Firm Foundations" won't trigger that ability (though they would still be upgraded if the land already had 2+ Explorers).
