I would like to throw out the idea of adding some token bonus to mana nodes in the event they are inside your city radius. Something like either two hammers (for earth and possibly others) or two commerce. This would at least make the tile workable. Since the AI treats mana nodes as a "resource" and thus tries its hardest to have a city witht he mana node inside, it would just make it more of a bonus than a detriment. On a related topic, would you consider changing water mana nodes to also be a source of fresh water. This would be a huge tactical advantage in areas with poor irrigation.