WaterKnights trick is very useful, however could you possibly just use a dummy spell based on channel with visible off to level up your ability? When dummy ability is leveled up, if level is less than 1, add real, otherwise skill up. And then just set the ability to max+1 TO hide real ability? That way you would never see it in the skill tree?
As I have explained this situation in my previous post.
The levelling of the spell would be messed up.
Also, the damage of the spell will also be messed up if the spell uses current level of the ability's level as point of reference such as damage, aoe, duration, etc.
Let me try to rephrase your statement.
You mean if your spell has 4 levels, you should make it as 5 levels, as last level would be the hidden visibility, right ?
So, if you have levelled up that ability up until Level 4, and you have 1 more Skill Point to use, when you open the Level Up Ability window, you will see that it has extra level - thus ruining the process.