I have uploaded 455 volumes of Russian WW1 casualties (in Russian language) to the Internet Archive. These books were obtained from several Russian library sources. The lists are provided by Russian province. Go to www.archive.org and search the term "memorial books".

In addition, there is a Russian website that has a project going to provide the data within pdf format spreadsheets. These are also in Russian and provided according to Russian province. You can find these records at http://svrt.ru/1914/1914-1.htm . I have not verified all 455 volumes are represented in these lists. In both the original lists above, as well as the indexing works, casualties only cover the years 1914 - 1915. 

I continually search the Russian library sites looking for lists covering the later years of the war however, as of Sept. 2017, these lists have not been located.