Ja som našiel jeden tip pre rxRichEdit, neskúšal som:

uses
  RichEdit;

// Stream Callback function
type
  TEditStreamCallBack = function(dwCookie: Longint; pbBuff: PByte;
    cb: Longint; var pcb: Longint): DWORD;
  stdcall;

  TEditStream = record
    dwCookie: Longint;
    dwError: Longint;
    pfnCallback: TEditStreamCallBack;
  end;

// RichEdit Type
type
  TMyRichEdit = TRxRichEdit;

// EditStreamInCallback callback function
function EditStreamInCallback(dwCookie: Longint; pbBuff: PByte;
  cb: Longint; var pcb: Longint): DWORD; stdcall;
  // by P. Below
var
  theStream: TStream;
  dataAvail: LongInt;
begin
  theStream := TStream(dwCookie);
  with theStream do
  begin
    dataAvail := Size - Position;
    Result := 0;
    if dataAvail <= cb then
    begin
      pcb := read(pbBuff^, dataAvail);
      if pcb <> dataAvail then
        Result := UINT(E_FAIL);
    end
    else
    begin
      pcb := read(pbBuff^, cb);
      if pcb <> cb then
        Result := UINT(E_FAIL);
    end;
  end;
end;

// Insert Stream into RichEdit
procedure PutRTFSelection(RichEdit: TMyRichEdit; SourceStream: TStream);
  // by P. Below
var
  EditStream: TEditStream;
begin
  with EditStream do
  begin
    dwCookie := Longint(SourceStream);
    dwError := 0;
    pfnCallback := EditStreamInCallBack;
  end;
  RichEdit.Perform(EM_STREAMIN, SF_RTF or SFF_SELECTION, Longint(@EditStream));
end;