I want to approach a certain subject for my diploma project, but unofficially I may get a bad grade for doing so. What should I do? I'll need to explain more just so my question can be properly understood.

I'm a 4th year Biomedical Engineering student from Romania, currently enrolled in the "Biomaterials and Medical Devices" engineering Specialisation. The previously mentioned specialization was indirectly chosen in the course of the 2nd year, when we were told to apply to either Biomaterials and Medical Devices specialization, either to Medical Equipment and Systems - with no additional information regarding the exact difference between the two, even when asked bout it. 
Due to the fact that I had several failed exams which did not have the time to retake until the specialization lists were made, I ended up in the BMD spec. even though I chosen the other one.

In the last two years we took part in courses regarding mostly Biomaterials and very to no little courses about medical devices (those being Introduction to Telemedicine, Medical Databases and sort of an Introduction to Medical Imaging) - the problem is the aforementioned medical devices disciplines were common; by that I mean that even though we chosen our specialization, 4 out of 12 discipline in the 3rd year were made by both BDM and ESM students.

Now being in the 4th year, I have to choose my diploma project subject in the way that it will reflect the things I have learned along these 4 years, with a strong focus on the last two, when the BMD specialization was chosen. Almost all my colleagues are approaching subject related to the synthesis, functionalization and characterization of Biomaterials, while I want to approach a subject related to medical devices. Since Romania is part of the European Union, it has somehow standardized its higher education up to a certain point to which students from Romania can apply their knowledge in other EU countries with no problem. With that being said, there is a EU Directive about the meaning of medical devices and all its subcategories, which states the following:


  Directive 2007/47/EC defines a medical device as (paraphrasing): Any instrument, apparatus, appliance, software, material or other article, whether used alone or in >combination, together with any accessories, including the software intended by its manufacturer to be used specifically for diagnostic and/or therapeutic purposes and >necessary for its proper application, intended by the manufacturer to be used for human beings for the purpose of:
  
  
  Diagnosis, prevention, monitoring, treatment, or alleviation of disease
  Diagnosis, monitoring, treatment, alleviation of, or compensation for an injury or handicap
  Investigation, replacement, or modification of the anatomy or of a physiological process
  Control of conception
  


Source: Wikipedia

But apparently, the faculty has been less than keen to apply these standards to the curriculum. Another problem would be that the evaluating commission would be made out of, let's say, 10 professors with 8 of them specialized on a different branch of biomaterials. Therefore, I have to choose a subject that these 8 professors would understand to, so they can grade me. Henceforward, no software, medical imaging or other subject like these ones. There was one student last year who presented a database of imaging results upon biomaterials which could be sorted using specific filters - and even though the presentation was correct and the database was up and running, he was given a much lower mark due to the fact that these professors did not understand the processes that made the DB work in the first place. After all these, the pro-dean has to approve my diploma project subject, before proceeding to it. The pro-dean is an ESM specialized professor and expects to see something related to biomaterials in it in order to approve it.

Should I go on and approach a non-biomaterial or almost-non-biomaterial subject as the description of my specialization give me the right to do, or just give up and find one related almot or entirely to biomaterials?
Your faculty presumably knows much more about the area, and if they suggest (formally or informally) that some line of inquiry isn't suitable for a thesis, I'd listen carefully. It might be that the task is just far too ambitious to really solve in reasonable time (I see that very often with my own students), or it sounds superficially interesting, but has already been solved or the solution (if found) won't be an advance. Sometimes the thema is worthwile, but the risk of not being able to complete it (or not getting results) is just too high.