Thank you very much, @marcel.arentz, again for your reply!
I see your points. We will discuss internally whether or not we will invest time in developing such a plugin ourselves.
I also thought of other kind of support we could provide like testing a new plugin in case tribe29 would take it up. Let us know in case you would start working on such a plugin and in case we could support you (e.g. in testing it).
Said this, is there any (other) way we can monitor an mssql instance running on Linux using CheckMK?