According to the wiki page on Rav Tzadok he says something about the historical development of halacha (though it would have nothing to do with making halacha more liberal.) Maybe they’re just extrapolating from that to think that they can therefore “advance” halacha as they see fit?
I don’t know of any chassidus that permits rewriting halacha (aside from the issue of davening times, but I think that was a matter of custom or practice rather than psak). I also think most OO writers do not use chassidus to justify their ideas — maybe it is just one or two authors?