The Zener diodes are used in the transformer in conjunction with the boost winding in the PW transformer, they're not a real solution to this issue.
I'm guessing with a bit more work you could build a box that minimized the speed change while still imparting a DC offset to the track, I never spent much time trying to do that.
Switching them in is pretty easy, just wire the switch to the NC contacts instead of the NO contacts. When you push the button, it'll insert the diodes into the line. For the box I pictures above, that would involved moving one wire on each switch to the opposite side. You'd still have the speed change, but it would tend to slow it down and not speed it up.
