Hmm...
I have been experimenting elsewhere with making a huge number of secrets, each with a single minor spell, basically so that scholars could spread useful information in a fort and collecting a library would allow you to accumulate more spells for your wizards. Unfortunately, it seems that the more custom secrets there are, the lower the chance of towers being made (I guess because each world generates a certain number of secrets picked from a list, and the more non-necro secrets there are, the less chance of necromancers showing up), and since necro towers are still the main places where wizards trade knowledge, the secrets don't seem to spread too well in worldgen. And if you *do* get a secret in a fort, it spreads like crazy, since all someone needs to do to learn a spell is to read a single book. (One possible solution is to create a dedicated wizard caste using a syndrome that adds the SUPERNATURAL tag, and prevent non-supernatural creatures from learning spells. I haven't actually tested how well this works in practice, though.)
If all or most secrets included the ability to produce minions from corpses, that might improve the ability for spells to spread. Unfortunately, it's pretty much impossible to make such a mage 'safe' for fort mode; transformed zombies will indeed build towers, but they will also attack the living just like regular zombies.