Currently it's not possible to bind any action to mouse buttons (aside from primary attack on left click and secondary attack on right click) unless the mouse you have comes with a dedicated software allowing you to change key bindings.
There are reasons why somebody would want that to change. For instance, some people got used to having jump be their right click from other games, and some people are comfortable having actions bound to their additional mouse buttons.
It is possible in about every recent game that I've played, so it shouldn't be too hard in theory. And as long as you don't include scroll down / scroll up (which would make IWJ trivial), it's not like people are going to gain an unfair advantage because of it.
I know it probably won't be very high in the priority list, but it's a big deal for me personally.
EDIT: it's actually possible to bind an action to scroll. I tried it with jump, and it does make IWJ extremely trivial, although since you can't bind an action to two keys simultaneously, it makes everything else difficult.