I feel like stating the obvious when I say: The proposal is just not how labels work.
They are a key:value pair, making several vales per key available breaks a lot of logic as @r.sander already pointed out. If you tell your kid ‘Your name is Gunther’ and one day later you call it by the name ‘Jørgen’, what do you expect? Confusion, correct.
Workarounds (which are not workarounds to me, but proper usage of the features at hand) have also been outlined.
Additionally, for some use cases here you can and should use tags instead of labels. Say you want to label a host regarding its database or web server: Create a host tag ‘webserver’ or ‘database’ and provide all products you are using as choices. That way, you make sure there are no typos and the information is represented consistently across your environment. And if you now want to say, what about two database systems on the same server, tags will not work then, let me tell you: That would be just bad practice. Putting more than one service, especially of the same kind on the same server is just poor IT architecture.
Maybe a friendlier bottom line: The labels feature is very powerful and thought through. Please try to think about different solutions to your ‘problem’, I am sure Checkmk can provide you with a lot of flexibility there. Also, you saw the collective intelligence here at work, so I am positive you will find a way to do what you want to do.