Synergy is an application I use to share my keyboard and between a VFIO guest and host system. One thing has bugged me about it for ages but today I found a solution that...

By first exporting your config and then selecting the use existing configuration section you're able to add a couple of key lines to the synergy config which aren't exposed via the UI.

section: options
        mousebutton(6) = keystroke(WWWBack)
        mousebutton(7) = keystroke(WWWForward)

There's probably a bunch of other stuff in the options section but by adding keystroke() we're able to finally use the forward / back mouse buttons in a Synergy client!