What you want is a per-class lock. That would partially solve the issue, and make everyone happier.
Bad idea and will never happen. Same with tome cap per job. Both ideas are counter intuitive to us being able to play all classes on one character.

Another problem is this, the people with more classes levelled get way more benefit out of it than those who don't have as many levelled. If you have four 50's you could run coil 4 times as much as someone who only has one 50. It's unbalanced and would just allows those with more time to get further and further ahead of other players. Considering that, you would need to be willing to have your drop rate divided by the amount of 50's you have.