I am also interested in this and actually have a semi working version (needs more testing) based on the original T5 repo. I’d be happy to work together on this to bring to the transformers library if it is still on the roadmap. Here is the colab with the current implementation: Google Colaboratory 65 (scroll down/CTRL-F for DataCollatorForSeq2SeqMaskLanguageModeling
I can also open a PR to start this process if interested.