Completely unrelated to my field (I do not work in languages) but I have written a small piece of software that proof reads my work for grammar and syntax. I'm interested in finding a few documents (ideally from a range of academic disciplines) that I can use to check my proof reading software.
If you perform a search for "examples of bad student papers" or "examples of bad writing" you should find many examples. As an added bonus, many of the examples are then analyzed to provide suggestions for improvements.
I am unclear how this would differ from the grammar and spell check in a word processing software such as Microsoft Word; but presume it must differ or you wouldn't have created it! Perhaps as an initial trial for proof of concept, you could compare how yours competes with Word or other similar programs. This would provide some preliminary data with something most researchers are familiar with so can relate to it. If it compares well, it would also allow you to begin telling the story of your tool in that context, which is valuable in gathering support, and even funding if there are grants out there for this you may want to seek. Next, (possibly the beta stage for the new grant!) you could design a study using professional human (teachers, scientists, etc.) reviewers to compare against. The student papers suggested in another post may provide material for this.