I made a Greek POS dictionary and the necessary files to integrate it with LanguageTool. I think it would be nice if it was included in LanguageTool by default. I'm willing to maintain all relevant files.
I should note though, that as the data is autogenerated, if there is an error, it should be reported and fixed on the original projects used.
LanguageTool data for greek is here: https://github.com/stevestavropoulos/elspell/tree/master/grammar/languagetool
Data is generated with this: https://github.com/stevestavropoulos/flexy with data from https://github.com/stevestavropoulos/elspell/tree/master/grammar (there is also a Makefile for help with the generation of data).
Also from https://github.com/stevestavropoulos/elspell you could get a more recent version of the spelling dictionary to use in LanguageTool.
All projects are mine and I can provide you with any license needed.
PS. I tried several times to subscribe to languagetool-devel, but with no success. I get a message "Your subscription request has been received, and will soon be acted upon", but no email is sent to me or anything else.
PS2. If someone is interested, flexy could be used to help generation of POS data for other languages as well, besides Greek.