It would require customization, or Shipping Director can do it.
RE: "dummy" [default] weight for products that don't haveSD has Packing
, and you can calculate a $PackageBoxWeight based on those products without weight.
RE: products over a certain weight do not have the xxx to free shipment
With SD, you'd disable the nopCommerce built-in 'free over x', and calculate the shipping rate. You can "pack" the products excluding
those that are free ship, and then calculated a rate (either by your own calculation, or call another rate plugin like ByWeight, FedEx, etc.).