Well, I had an idea that would incorporate the towers with the metamagic spell sphere, so that each tower (and the Amurite Palace) would provide 1 mana source, and the only way to get all the metamagic spells would be to get a mana source for each level.
The node-making spell could fit in as one of those metamagic spells (Level 2 or 3, maybe?). That would limit it more to players who are trying to get the Tower of Mastery rather than those who are trying to getting Fire II for all their mages for free. With the default 3 mana a civ gets, and 2-3 mana sources, building two of the towers should be easy enough.