The example code above requires my Rcom.c header. I can submit it (I think there is a place for user contributed code, besides pasting to the chat?) if anyone is interested.

My solution is adequate for my purposes. The only drawback to writing to a JSON file that I can see is that it is slow.

It actually is possible for R to store strings as double vectors, since R has access to C via Rcpp. Though I personally don't have the motivation to implement this, since it would take a bit of work.