Dimension are not specially attached to MKS (or any other systems: you can find them in MKSA, etc.) since they are defined without the reference to a system (contrary to a unit). The aim is to pack them into a system in order to avoid to have a lot of variables into the namespace (as we can see in some existing modules). So the systems are used to gather everything you need, instead of having all defined randomly in modules.
But if you look into mks.py you will see that length, etc. are also defined in them and you can import them. And they are strictly equal to the one defined in the system. It could be easy to extend the dict method to accept also Dimension, but then it is not really useful since you get exactly the same object (and I stress it but the idea was to avoid to define non-wanted name in the namespace).
Also your DimensionSymbol exists already as Dimension in dimensions.py.