I can direct you to A Student's Introduction to English Grammar by Huddleston & Pullum, which on pages 185/186 says:
R element (relativized element) within an embedded clause
It is possible for the R element to be located within a content clause embedded inside the relative clause:
 i a. a key [which he says she found] b. a key [(that) he says she found]
ii a. some boys [who he says saw her] b. some boys [(that) he says saw her]
In [i] R is object of found, and the found clause is a content clause functioning as complement of says: "He says she found R". We understand that he says she found some key, and that's the key the whole NP refers to.
In [ii] R is subject of the embedded saw clause: "He says R saw her". Note that the that is omissible in [iib] : this differs from [6i] above (some friends that saw her) in that the R element is subject not of the relative clause itself but of the content clause embedded within it.
I cannot say thank you enough for providing this citation! It was precisely what I needed in order to locate all the applicable sections in both of the references I mentioned (prior to the additional confirmation provided by Robby zhu). In case it is of interest, though, (since I see you do not have access to either of the tomes I asked about), would you like to have them for your own research?