If you can not mock-up an instrument convincingly in a piece (that eventually will end up without recording of real instruments in a score) due to the lack of decent libraries or just out of the fact that this instrument can not be simulated convincingly, you should rather leave it out and replace it by something else.
Even if the rest of your mockup is convincing, just one element that sounds like “plastic” will diminish the overal effect of your production. Generally, when you need to write a score that will only be produced with samples, don’t be too adventurous on your writing but rather know what (your) samples are capable of and write as effectively as possible for them. Even if your orchestration is stellar and would sound fabulous with real instruments, it will not be capable of compensating a mockup that sounds partially fake because your adventurous orchestration can not be reproduced with samples.
Clients often find music bad because it’s not sounding convincing to them and often don’t find things as bad that sound convincing but might not be as detailed in orchestration.