Its easy, IF you look at this the right way.
First, pre-parse the expression to determine that 'a+b.*c' has three variables in it, a, b, and c. Define objects a, b and c. Now overload the plus and times operators to actually use bsxfun.
Finally, let MATLAB do the operation, using an eval. You CAN make this work, but you will need to be careful and think it out.