This posting is older than 6 months and can contain outdated information.
Thanks for asking.
If you want to emulate a space bar press with a particular button, you can assign it to a button at
Prefs > Mapping > [Browser of choice] > [Button of choice]
Virtual Keyboard > Space(bar)
for that button. There's no guarantee, though, that this space bar press ends up controlling an embedded web video, though. Depending on input focus, it could also end up f.ex. adding a space character to the address or search fields of the browser.