Hey ST, glad to hear that you are taking care of business.
In DC, can't memorize means can't memorize whenever the spell is given to the spellcaster, under any circumstances.
With that said, you almost gave the way to do it in your question - make it unlearnable by characters - then the spellcaster will not have the spell unless you give it to her.
Unfortunately, just using this trick does not specialist mages (or priests) make. The designer (you) may intend that certain spells only be give to certain characters, but a player could still create a mage and give them spells from all of the 'special' groups that you have created. Now, with that being said, it may be possible to do this using special abilities - you could have a player choose which specialist mage they want for a partiuclar PC and a special ability is assigned to that PC. Now, you will have added an appropriate SA or flag on each spell that will allow it to only be used by a spellcasting PC with the correct SA (given in the first part). I think this oculd be done right now, but it would be a lot of work - adding an SA to each spell and creating ones to be given to PCs - not hard at all, just a lot of work.
