Instead of changing the Layer intersection priority and updating all the layer combinations (which could be quite a big job just to work around a couple of instances where this problem occurs!), you could try just duplicating the slab in the same position, placing it on the SEO layer (which 'should' be on a completely different layer intersection priority number anyway to avoid unwanted interaction with visible elements), then SEOing the slab from the beams.
When the SEO layer is then turned off you should get the required effect of the slab appearing to cut the beams away.
Not quite automatic, but the best I can think of!