Hi,
KeyedSet adds a named instance variable to a class that Magma treats as indexed, so it is not supported. It definitely can be, and if you wish to work on it I would be grateful. But it will require some careful changes to "Ma object serialization" package and probably "Ma traverse object graphs" too.
The framework is already in Magma - there are just some cases to iron out in the serialisation of these instances.
I can help with advise if Keith/anyone is willing to pursue it.
Brent