After switching from the old RCurl (which required repairing unescaped whitespace) to the more current, R-escaping-friendly httr, I could keep an eye on the crul-beta. The replacement code would be:

x <- crul::HttpClient$new(URL, auth = crul::auth(cred[1], cred[2]))$get()
jsonlite::fromJSON(x$parse("UTF-8"))