Hi @MattP,
your links provide the answer.
Use the code snippet from NTLM authentication in httr2 - #2 by jsmfws
and set httpauth = 2L. For a listing of the methods from httr you can look at httr/R/authenticate.r at main · r-lib/httr · GitHub.
So keeping @jsmfws style it would be something like
req_auth_digest <- function(req) {
httr2::check_request(req)
httr2::req_options(req, httpauth = 2L, userpwd = "username:password")
}