Rhetorical questions with "used to"
Didn't he use to write really well?!
There is some debate around whether it should be "didn't he use to" and "didn't he used to" which is discussed at length in Are there alternatives to the common construction “didn't used to”? The short version is that "didn't use" to is more commonly considered correct.
But yes, this usage would be understood and is spoken rather frequently.
As for "usedn't", it certainly wouldn't be mistaken for actual English so readers would have to assume you were playing around.