How would a collective recommendation letter from co-workers be perceived for admissions to MFA requiring work experience?
I had a "co-written" letter of recommendation when I was applying to graduate schools. I thought it was fine, and it was also somewhat unique, as did the people reviewing the application. The interviewer for one of the programs to which I was applying said that in two decades, he had never gotten such a letter before. So your letter will definitely stand out. However, there was also the comment that the letter was a bit strange and came across a bit "gimmicky."

I might recommend instead that you have one of your colleagues "officially" write the letter, and have the letter state that his co-workers (and name them!) also contributed to the content of the letter.