I am writing an implementation of this srfi intended for gambit, but code is mostly r7rs.
link: https://github.com/alejsan/gambit-srfi-180-json

I have a few questions and comments.

1. Should the generator produced by json-generator close over the current values
of json-nesting-depth-limit and json-number-of-character-limit when it is
created? Consider the example:

;; generator created, default parameter values of +inf.0
(define gen (with-input-from-string "true" json-generator))

;; generator used, reference implementation does not raise an error
(parameterize ((json-number-of-character-limit 1))
  (gen))

2. Why is the port-or-accumulator argument to json-accumulator required? I think
the returned accumulator should default to writing to the current-output-port
just like json-write does.

I believe the following are errors in the reference implementation:

1. The reference implementation of json-generator accepts ONLY an input-port and
does not accept a character generator as its argument.

2. The reference implementation of json-accumulator accepts ONLY an accumulator
and does not accept an output-port as its argument.

3. In the reference implementation the accumulator produced by json-accumulator
expects its input to be pairs such as (cons 'json-value 10) or (cons
'json-structure 'array-start). It does not accept the output of generators
produced by json-generator.