The Matlab Filter is still marked as "unstable", due to some limitations.
One of them is the current input/output structure, limited to streamed matrix and stimulations.
We plan to rework this box in the future roadmap, but I can't be more precise about when and how.
Anyway, it could be a very nice contribution for the community !
If you want to modify the box by adding more input and output yourself, you will need to patch the source code of the box.
The code can be found in openvibe-plugins/matlab/trunc/src/box-algorithms/tools
I don't know how familiar you are with C++ in general and OpenViBE development in particular, but this task may not be easy for the new comer.
The developer tutorials
can help you on this task.
Follow us on twitter >> openvibebci
Checkout my (old) blog for some OpenViBE tips & tricks : here !