Open data is certainly an important movement as in the future is will be easy for huge groups of users to collect data.
Mendix is a commercial tool, often used to support business processes. It is currently not available for open projects.
Edit: To be more precise, tools for open data should be open as well and preferably free, IMHO, and Mendix does not fall in that category.
Edit: you make your model you can use it almost one on one in Mendix. Two (or more associations) between two entities are allowed. Often used in Mendix as 'selectable' (reference set) and 'selected' (reference)
I think in a way Mendix does support open data. The button to create Excel or CSV exports are there and webservices can be used to make the data available to other machines. So the first three stars of the 5 star model are reached.
To make my question more precise.
What I would like to do is to model a semantic model (see link below)
The bullets are concepts, the arrows are relations.
It is possible to have multiple arrows between two concepts.