I read Sun Yatsen‘s claim that 民權 as a noun had entered the Chinese language in the recent past (近代 usually denotes times after the opium war). He adds of course evidence that 孔子 and 孟子 expressed sentiments along the lines of 民權。
But I asked myself, what we can ascertain about the time of appearance of 民權 as a noun. We will almost certainly not find the very first user. But we may have evidence of people finding it unfamiliar, calling it a new word or the like.
The first mention I found was in Zhang Taiyan‘s 訄書，as he discusses the received opinion on Shang Yang. That is from about 1899.