I don't think that anyone alive still refers to themselves as /warawa/, which is the first-person pronoun* denoted by 妾. You might find it (or some cognate) in use by the very old or those speaking non-standard dialects (or other Japonic languages), but I think you can safely call it extinct in "Standard Japanese".
In a modern context I would be surprised to see 妾 used at all, but I guess /mekake/ "mistress" seems more likely than the other readings, if only because /mekake/ remained in common use longer (to judge from my non-rigorous sampling of Japanese literature).
You don't specifically ask this, but if you were also wondering why 妾 came into use at all to write a first-person pronoun... I don't know all the details, but here's what I do know. /warawa/ is a very old (Man'yoshu old) native Japanese word meaning "child". I forget the specifics, but I recall that it was used for children who were old enough to walk around, talk, etc., but still not yet adults mentally or physically. I guess "late elementary school" is about the right image. This was the meaning that was borrowed for the first person pronoun, so the meaning is very humble.
The "standard" character for /warawa/ "child" was 童. I would imagine that 妾 was used for the female first-person pronoun because 妾 is the "definitely female" version of 童. I doubt that the "mistress" meaning was a factor; it might not even have been current in Japan then (I don't know much about the history of the character).
You probably know this already, but /boku/ 僕 is kind of similar: depending on how extreme you want to be, you could argue that it also means "servant" or even "slave". They took humbleness seriously in pre-modern Japan!
* Let's just stipulate that Japanese pronouns actually exist.
