Thanks very much, Mike and Andrew - those are very interesting leads, and exactly the kind of things (and references!) I was wishing for. Upon further reflection, the MATLAB integration is probably of secondary importance, so something like PMR2 would better work for model development and improvement (though a resulting SBML file could still be imported to MATLAB for simulation if so desired).
PMR2 looks like a great tool. I wonder what would be required to develop a similar platform for distributed development of SBML models?