I should think it’s possible but would probably require a bunch of experimentation and examination of the sample bloom effect in the engine.
Normally, you work with LDR colors but if you push them over 1.0, you could just bloom those colors. So if the bloom threshold was over 1.0, even a white background wouldn’t bloom.
Alternatively, you could render the bloomed scene to an offscreen render target and then composite that over a white background. That’s a pretty advanced thing to do in PlayCanvas at the moment, so you’d have to be comfortable with coding with the pc.Layer API.