I have a couple of live edge slabs I was planning on making into live edge end tables but I'm wondering what to do with the cracks in the side - if anything can be done at all. Could I fill them with a glue/sawdust mixture? Would some other commercial product be better? Is it worth attempting at all?
Could I fill them with a glue/sawdust mixture?
You can yes but common woodworking glue (PVA-type, white or yellow) wouldn't necessarily be the best choice for the glue.
A much more solid fill can be made by using epoxy. Filled (or just coloured) epoxy is now a common material for filling voids and defects in wood. To a degree this kind of filler can stabilise a crack and add strength, but without some further means to hold the wood steady (e.g. a butterfly key, hidden bolt or dowelling) a crack may continue to open up, so it is worth taking at least one extra step as a safety measure.