inputs.json

[
  {
    "func": "metadata",
    "value": {
      "code": "x = 1 + 1",
      "id": "a"
    },
    "label": "<r-chunk>"
  },
  {
    "func": "metadata",
    "value": {
      "code": "y = 2 + 2",
      "id": "b"
    },
    "label": "<r-chunk>"
  }
]

knit.typ

#let code(x, id: "a") = {
  [#metadata((code: x.text, id: id)) #label("r-chunk")]
}

With this code,

#code(`x = 1 + 1`, id: "a")

or another

#code(`y = 2 + 2`, id: "b")

#let output = {
  if "results" in sys.inputs.keys() {
    json(sys.inputs.results)
  } else { // default values
   (x: 3, y: 4,)
  }
}

`x` is #output.x

`y` is #output.y

results.json

{"x":2,"y":4}

test.R

library(jsonlite)
library(evaluate)
library(purrr)

inputs <- jsonlite::read_json("inputs.json")
e <- new.env()
walk(inputs, \(.x)  evaluate(.x[["value"]][["code"]], envir = e))
jsonlite::write_json(as.list(e), 'results.json', auto_unbox=TRUE)

typst query --pretty knit.typ "<r-chunk>" > inputs.json
R -f test.R
typst compile knit.typ --input results="results.json"