So far, R7RS has been incredibly easy to implement on top of #guile thanks because it is mostly R6RS and things that are not part of R6RS are in SRFI libraries that are well coded and portable.
I had similar experience getting together #R7RS in Chez #Scheme.
I know devil is the detail. Eventually, I guess that I will hit roadblocks that will require some patches in guile like `define-library` form.
Happy hacking!