I'm glad to announce that we now have a student working on a Master thesis that will be useful for LT: Fraser Bowen is a student of computational linguistics at Saarland University, and the basic idea of his thesis is this: use a modern machine translation framework to correct errors. Instead of translating from one language to another language, "translate" from incorrect English to correct English. All the low-level machine learning is thus done by the machine translation framework, and one "only" needs to generate a large amount of incorrect example sentences with their corrections.
Once we get this working for English, it shouldn't be too difficult to use the same approach for other languages.
Fraser, welcome to LanguageTool, we're looking forward to working together.