Have any writings or fragments of ante-Nicene Church Fathers previously unknown (that is the actual words were not known, not necessarily that the name of the work was also previously unknown) to modern readers been discovered within the last 20 years or so? If not, when was the last time such a writing was discovered?
Yes. 29 homilies of Origen were discovered in 2012 in the Bavarian State Library in Germany. Some of them were already available in Latin translation, but others were completely new discoveries. Origen scholar Lorenzo Perrone judged them to be genuine.
In 2015 the homilies were published in a critical edition, the front matter of which is available on academia.edu.
Several years prior, in 2008, six sermons of Augustine were discovered at Erfurt, in a 12th century manuscript. Larger groups of Augustine's writings had also been discovered in 1974 and 1990.
The Second International Conference on Patristic Studies will meet in Argentina in March 2017 with the purpose of discussing “Late Antiquity manuscripts found during the 20th and 21st centuries.” These include “the Nag Hammadi Gnostic library, the Dead Sea Scrolls, Origen of Alexandria’s homilies, Augustine’s sermons, etc.,” so the 21st century discoveries they reference might well include newly-discovered literature from the ante-Nicene period.
Inquiries to The North American Patristics Society's Facebook page or Stack Exchange’s History site might offer better access to expertise in this field. Good luck!