Gigabit detection DIY
No, that would not work. Even if you ignore completely messing up what is basically an RF link by introducing a nonlinear element, it just has not way to work. You have to monitor actual traffic to do what you want to do.

An ethernet link is basically negotiated at the physical layer when the link is initiated. If the physical layer sees that the two additional pairs cannot be used it would not just try to use them if the link becomes congested.

The additional pairs are not like a overspill gate on a dam. You either have a Gigabit ethernet link or you don't. You either use the extra pairs, or you don't. There is no in-between.