Boulez's time signature 3/4/2 is baffling and completely unique. Do mean to say that any kind of deviation is illegitimate?
I myself, have a particular affinity for this sort of thing because I have played a ton of music by living composers, where it often seems every piece requires a slight deviation from the 'norms'.
I am not sure what 3/4/2 means. If you mean 3+4/2, its old hat. Time signatures do not have the same qualities as key signatures. You could have 19+2.6/4.
I hope I am not coming off as argumentative, I am enjoying the debate

I play tons of new stuff too.
Answer this:
How many lines does a staff have?
If you answer is other than 5, then we are talking about apples and oranges here. I am talking about the notation of western music. I could put a banana peel on a music stand and call it notation, but that is not what ( I think) we are talking about.
What you are trying to say is the alphabet could be ABCDE2G etc. Fine, I can accept a new alphabet. What I cannot accept is having a debate on the rules of the alphabet without having any rules, the alphabet is ABCDEF etc, there is no "2" in the alphabet.
Modern Notation software is what composers use. Last nite I played a piece by R. Murray Schaffer, prominent living composer from Canada. It seemed to be written on Finale: There is NO WAY to write 8 sharps in Finale, I own it. I own Sibelius and Encore and Cakewalk too, only 7 sharps. There is a reason for this. Much like how a staff in Finale will only have 5 lines. Try to do it, you cant, I just tried. I have been trying for 10 years

it wont do it. It's against the rules.
Since you play cello, you know about alternative tunings (scordatura?). There is no FIXED law on tuning, although 99.999999999999999% of tunes have you tune the "normal" way.
There is a "fixed" law in regards to Key signatures, just like there is a "fixed" law with the alphabet. A computer keyboard cannot type a combination of "a" and "b", its one or the other.
If you WANT to put 8 sharps in a key signature, then you can put 6 lines in a staff. If this is what you are talking about than there can be no debate. There are 7 notes in a major scale: if you want to say there are 6,437 that's fine, but there are 7. Black and white, end of story, that's all she wrote, done deal, finito, case closed, done. THEREFORE, there can only be 7 accidentals. You can call it semantics all you want, there are 7. A sharp, double-sharp, triple-sharp, quadraple sharp, a sharp to the 5th power, whatever: you only get 7 per scale.