I know it's kinda been done before, but I have been commissioned to write a tool to convert an LXF model from LDD into a DAE file, and the commissioner is fine with me open-sourcing the project now that it's complete, so I might as well share it.
I give you, LDDCollada
Quick Notes:
LDDCollada is the command-line version, written in C# so use Windows or Mono. Drag and drop input files onto .exe to convert
LDDCollada.Mac is a macOS GUI that exposes options the CLI doesn't
Does not extract .LIF archives for you
Will generate properly-filled versions of the decal textures
Make sure your software is gamma correct - in Maya (first screenshot below), the filled textures look natural against the solid triangles, but in macOS Preview and Quick Look (second and third screenshots), the solid brick colors look dark but the decal textures look bright.
My first go at writing Mac software (I kinda cheated by using Xamrin.Mac)
Could be interesting if you want info on C# .g file parsing
Ignores normals
Screenshots: