I don't think that would work so well. Aside from a change in the modulus of the ABS base resin, the butylated melamine would disrupt the thermoplastic nature of the ABS when the BM thermosets. The overall rigidity and resilience that ABS brings to the table would be greatly reduced by the admixture. There would also be a change to the volumetric properties of the combined resins which would lead to inconsistent dimensional stability and destroy the necessary high dimensional tolerances needed for Lego to interlock and stay firmly connected. The thermoplastic nature of ABS and its elastic qualities give Lego its signature "snap" fit and friction locking abilities. I don't see the introduction of a melamine as anything but disruptive to this quality. Also, as you mentioned, the tooling would need to adapt to the material. I can see a need to adjust the draft angles of the bricks to properly eject from the molds and the need to adjust mold/part cooling profiles to accommodate for the new resin admixture physical properties. Sure Lego is brilliantly designed to hide the draft angles on the inside of the bricks and set the parting line at the bottom plane, but this might be for not after heating the bricks to thermoset them. The variance in the wall thickness caused by the drat angle will warp the bricks as they heat and set. I just see a brittle, dimensionally irregular and ultimate failure of a Lego brick coming from this "cheat". It will not solve any problems and will create many new ones instead. Might as well just glass fill the bricks instead and maintain some of the ABS properties while using a little less petro-based polymer in the process. They'd be shitty bricks like that too, but at least the tooling wouldn't have to change much.