the term 'epoxy resin' covers a variety of different formulas, in most cases it's the hardener that will be more noxious and cause the most concern. When properly cured, the hardener and resin will have formed a cross linked polymer which isn't toxic - it's not uncommon in chemistry for two harmful chemicals to react together and form a safe one. The safety warnings on manufacturers data sheets refer to the resin and hardener before they've reacted together and I suspect that a lot of the panic regarding using epoxy to repair mouthpieces is due to people not understanding that resin and hardener form a different chemical structure once they've reacted together fully which has different properties to the two reactants.
Worries about epoxy 'toxicity' seem to mainly be due to the possibility of mixing more hardener than resin and there being residual hardener leeching out.
In the case of a mouthpiece repair, the amount of epoxy would be small and the amount of potential hardener leeching out would also be tiny.
It should be noted that not all epoxies are toxic or harmful, some metal cans are coated with epoxy to prevent corrosion from acidic foodstuffs like tinned tomatoes, epoxy formulas are used to line some types of water pipes and epoxy resins are used in medical procedures like hip joint replacement.
The particular formulas of hardener that are least pleasant are the bisphenol A and amine based ones. Aliphatic epoxy is the safer type. It's difficult to find out much about the formulation of specific brands of epoxy, but JB Weld say that theirs is non toxic on their website. I think Mojo Bari uses JB Weld on his mouthpiece repairs and modifications, so maybe you could contact him, or you could contact JB Weld.
From what I gather, most adhesives and paints for sale to the general public have to comply with certain safety regulations, hence the profusion of water based adhesives in DIY stores.