It depends:

You can have mirrors everywhere, also on MDLs, when you write your own mirror effect, using environment mapping or a shader. Many water mirrors by users were done this way.

However the engine's built-in mirror method uses the MIRROR flag, which is a block surface flag and thus naturally only available for blocks, not for MDLs.

We will abandon the MIRROR flag in the future, and use only effects and shaders for mirrors. They are a lot more flexible and work on any surface.