if you search for variable rake forks, I think people have done both vertically slotted and horizontally slotted dropouts. If you really wanted to do it on the fly, you could re-purpose some cambio corsa rear drops and the adjustment mechanism. Making a fork that allows different rakes is on my list, but it has been on my list for quite some time.

I posted this because it seems like people regularly ask for recommendations for re-raking forks. John's price is more than reasonable, $20 plus shipping.