Pre Windows existuje toto:

uses Windows;

procedure TfrmMain.FormCreate(Sender: TObject);
begin
  AddFontResource('xxx.ttf');
  SendMessage(HWND_BROADCAST, WM_FONTCHANGE, 0, 0);
end;

procedure TfrmMain.FormDestroy(Sender: TObject);
begin
  RemoveFontResource(PChar('xxx.ttf'));
  SendMessage(HWND_BROADCAST, WM_FONTCHANGE, 0, 0);
end;