I think the majority of the body of literature written by Egyptians is written in formal Arabic. I simply come to this conclusion because as an avid reader I must have come across only one or two literary pieces written in Egyptian Arabic as 'pioneering experimental' works (as one author called his stuff).
For the sake of correctness, I should state that my field of reading includes history, science, political analysis and generally non-fiction topics, I have just been reminded that some poets that use Egyptian as their writing language (it is a sub-genre called A'mmy), however they didnt have a standard agreed-upon way of writing it too.