Launching the Zoom app from Wavebox does not provide the option for virtual backgrounds:
Using Ungoogled Chromium, the options are available:
It's suggested here that the Virtual Backgrounds option may only be available for some browsers. As I understand that Wavbebox is a Chromium-based browser, I would have expected this to work. Perhaps there are some settings that need to be changed on the Wavebox app, but I could not find any. I did try to turn off hardware acceleration in Wavebox, just in case, but it had no effect.
So I'm left with not being able to use the Zoom web app through Wavebox. As the Zoom desktop app is awful on Linux, I would prefer that not to be a solution.