ok, I have been looking for an answer to this question:
How can one entity contain several methods in BDC?
The answer is to define a MethodInstance with a type of GenericInvoker, this enables you to define a method on your entity which can be accessed via code but that is not exposed for the OOB BDC webparts. See http://agiledirect.wordpress.com/2007/08/09/bdc-writing-to-bdc-using-genericinvoker/ for an example.
You can also use the type ViewAccessor to return a different set of data which can be used in the OOB BDC webparts. See http://znetsys.spaces.live.com/blog/cns!ED65D09C41E55E06!115.entry for an example.