Should I include a recommendation letter in my MS applications if it is not required? I am considering my options for further Master-level studies in Europe (the goal being to improve my PhD application down the line). To summarize, my current transcript is weak, but I may be able to get good recommendations from one or two professors or researchers. This could remove some doubts about my academic abilities, and confirm some past circumstances that explain why I didn't perform better, so I see it as quite crucial in my applications.

One university I am considering states that admissions are based on "past grades, and relevant experience (including work, research, or anything related to the field)" (paraphrasing). Unless I missed some hidden meaning, this excludes recommendation letters.

I imagine that I will be able to provide supporting documents later in the process. Should I then ask for recommendation letters and provide them anyway?

I will of course contact the university directly about this, but I would like to know if there is a more general etiquette for this situation. Do admission officers generally see a good recommendation as positive even if not specifically asked for? Can they on the contrary see it as an attempt to hide other weak points in the application?
Generally speaking, any material that is not asked for will not be considered in an application for a postgraduate degree. Admissions procedures attempt to create a level playing field for applicants as well as minimise the amount of time it takes to evaluate an application, and extras that aren't asked for are tossed in the bin. Some places may treat that material differently, but you asked for general.

Now speaking specifically, I would be surprised if you were not asked for recommendations during the admissions process. The line you posted does not preclude recommendations, rather it might reveal what elements of a recommendation letter they will pay attention to. You need to either start the admission process online and see what documents you are asked for or call the admissions office for clarification.