b) Those which retrofitted R6RS into an existing implementation but did not contribute: Guile.
c) Those written after R6RS that (I think) were concerned to implement the latest standard: Ikarus/Vicare, Iron, Loko, Mosh, Sagittarius, Ypsilon. In addition, Biwa implements only the base library and a handful of others.
From which I think we may conclude that retrofitting R6RS is not a popular thing to do.
R6RS is quite a big language and there are already so many good implementations of it that new ones might not have much to add. Same situation as with Common Lisp. R7RS is such a small language that all kinds of niche implementations are interesting and useful to explore.
Curiously, I can't think of a single R6RS->C compiler. There are interpreters and a few native-code compilers, but no compile-via-C.