Ok, desdeminha ultima pergunta não obteve respostas, estou avançando em uma direção diferente. Ri muito!

Não consigo encontrar nenhum exemplo aléma documentação oficial no gerenciamento do estado do usuário ou no acesso aos resultados de um analisador anterior.

N.b. Este código faznão compilar.

namespace MultipartMIMEParser

open FParsec
open System.IO


type Header = { name  : string
              ; value : string
              ; addl  : (string * string) list option }

type Content = Content of string
             | Post of Post list
and Post = { headers : Header list
           ; content : Content }

type private UserState = { Boundary : string }
  with static member Default = { Boundary="" }


module internal P =
  let ($) f x = f x
  let undefined = failwith "Undefined."
  let ascii = System.Text.Encoding.ASCII
  let str cs = System.String.Concat (cs:char list)

  let makeHeader ((n,v),nvps) = { name=n; value=v; addl=nvps}

  let runP p s = match runParserOnStream p UserState.Default "" s ascii with
                 | Success (r,_,_) -> r
                 | Failure (e,_,_) -> failwith (sprintf "%A" e)

  let blankField = parray 2 newline

  let delimited d e =
      let pEnd = preturn () .>> e
      let part = spaces >>. (manyTill $ noneOf d $ (attempt (preturn () .>> pstring d) <|> pEnd)) |>> str
       in part .>>. part

  let delimited3 firstDelimiter secondDelimiter thirdDelimiter endMarker =
      delimited firstDelimiter endMarker
      .>>. opt (many (delimited secondDelimiter endMarker
                      >>. delimited thirdDelimiter endMarker))

  // TODO: This is the parser I'm asking about.
  let pHeader =
      let includesBoundary s = undefined
      let setBoundary b = { Boundary=b }
       in delimited3 ":" ";" "=" blankField
          |>> makeHeader
          >>. fun stream -> if includesBoundary // How do I access the output from makeHeader here?
                            then stream.UserState <- setBoundary b // I need b to be read from the output of makeHeader.
                                 Reply ()
                            else Reply ()

  let pHeaders = manyTill pHeader $ attempt (preturn () .>> blankField)

  // N.b. This is the mess I'm currently wrestling with. It does not compile, and is
  // not sound yet.
  let rec pContent boundary =
      match boundary with
      | "" -> // Content is text.
              let line = restOfLine false
               in pipe2 pHeaders (manyTill line $ attempt (preturn () .>> blankField))
                  $ fun h c -> { headers=h
                               ; content=Content $ System.String.Join (System.Environment.NewLine,c) }
      | _  -> // Content contains boundaries.
              let b = "--"+boundary
              let p = pipe2 pHeaders (pContent b) $ fun h c -> { headers=h; content=c }
               in skipString b >>. manyTill p (attempt (preturn () .>> blankField))

  let pStream = runP (pipe2 pHeaders pContent $ fun h c -> { headers=h; content=c })


type MParser (s:Stream) =
  let r = P.pStream s

  let findHeader name =
      match r.headers |> List.tryFind (fun h -> h.name.ToLower() = name) with
      | Some h -> h.value
      | None   -> ""

  member p.Boundary =
    let isBoundary ((s:string),_) = s.ToLower() = "boundary"
    let header = r.headers
                 |> List.tryFind (fun h -> if h.addl.IsSome
                                           then h.addl.Value |> List.exists isBoundary
                                           else false)
     in match header with
        | Some h -> h.addl.Value |> List.find isBoundary |> snd
        | None   -> ""
  member p.ContentID = findHeader "content-id"
  member p.ContentLocation = findHeader "content-location"
  member p.ContentSubtype = findHeader "type"
  member p.ContentTransferEncoding = findHeader "content-transfer-encoding"
  member p.ContentType = findHeader "content-type"
  member p.Content = r.content
  member p.Headers = r.headers
  member p.MessageID = findHeader "message-id"
  member p.MimeVersion = findHeader "mime-version"

Um exemplo truncado do POST que estou tentando analisar a seguir:

content-type: Multipart/related; boundary="RN-Http-Body-Boundary"; type="multipart/related"

--RN-Http-Body-Boundary
Message-ID: <25845033.1160080657073.JavaMail.webmethods@exshaw>
Mime-Version: 1.0
Content-Type: multipart/related; type="application/xml";
  boundary="----=_Part_235_11184805.1160080657052"

------=_Part_235_11184805.1160080657052
Content-Type: Application/XML
Content-Transfer-Encoding: binary
Content-Location: RN-Preamble
Content-ID: <1430586.1160080657050.JavaMail.webmethods@exshaw>

XML document begins here...