Hello @ErichStyger ,
thank you for your article and answer. I've just realized that you are the person that write all those good articles on mcuoneclipse!! It's an honour and a pleasure. I've recently find out about MCUExpresso Working Set following you indications, and others.
Concerning the library, I finished yesterday and it works! It's been a bit demanding because I had to convert a lot of #defines into consts and extern consts, so that library constants can be configured in the application.
For the library I've followed this one: https://community.nxp.com/t5/LPCXpresso-IDE-FAQs/Creating-and-Linking-to-Library-Projects/m-p/469212. Did you know it?
I had to start a brand new project since, as far as I understood, it's the only way to have a static library.

Trying to change the Build Artifact for an existing project, the only possibilities are: Shared Library and executable.
However I'm going to read your article for checking how you would do it. Thank you again for your work, highly appreciate!