I like the idea. However, I currently have no idea how to track a tag skill.
I believe the http://geck.gamesas.com/index.php/IsPlayerTagSkill function returns 1 if the specified skill has been tagged. I just wanted to alert you to that function in case you didn't know about it.
One suggestion I have if you start to get bored with working on your script is to listen to some music if you like any, at least that is what works for me. Usually I go for the music that has a good beat to it, because most of the time that gets me a lot more active. Good luck with your mod.
Late edit: Forgive me if I am mistaken, but if the Player invests a skill point past your cap won't the Player lose that skill point forever? If not, then do not bother reading further, I am just embarrassing myself.
I was thinking for a way around this, and I think I have an idea. Every time your script reduces a Player's skill by one, it could also increase a global or quest variable by one. Then you could add a perk to the Player, that uses that global or quest variable as a condition. If the condition checks the variable and it is equal to one, then a "Adjust Gained Skill Points" perk entry would add one skill point to the next level up and so on.
It would require a perk entry for each number of increased skill points however, like this picture hopefully describes: http://img5.imageshack.us/img5/4982/examplepicture01.jpg
You could then reset the variable to zero after the Player levels up, perhaps you can detect that by using the http://cs.elderscrolls.com/constwiki/index.php/GetLevel function.
With this method theoretically when the Player accidentally adds skills over the cap, the lost skill points just gets added to the next level up. However I have not tested this myself, so I am not sure if it will work.