If I understand correctly, you're currently preventing LDS shuffle by having the LDS recipe output "molten LDS", which you then have to solidify into real LDS. I think you should also be able to accomplish the same goal by adding a "molten plastic" recipe, and then changing the LDS recipe to take molten iron/molten copper/molten plastic and output solid LDS. This feels a little less arbitrary than having a "molten LDS"; I don't know if it's a huge improvement, though.

Ah, I bet you can't hack 3 fluids into the foundry though, can you? That's too bad - disregard this, then.

Exactly, the foundry only has 2 fluidboxes, so you can't have 3 fluids in a foundry recipe. The molten plastic route would also require adding more recipes than I would like.