Most 8xC51xx ICE's are based on on standard IC's, operating in an undocumented mode called "Enhanced Hooks Emulation Mode". Philips, Infineon and Temic commercial 8xC51xx (or C500) CPU's implement this mode. This mode is selected during Reset transition, in combination with the state of some other pins (ALE, PSEN,EA and maybe Px.y pins). I have no other info but I am looking also on it.
As I know this mode is a patent of Metalink (ICE manufacturer)