I have read that Ceibo ICEs use a Philips MCU in some special mode for emulation (with some security bit(s) activated).
In other place (maybe an Atmel's manual?) I have read that ONCE mode simply left the ports in a hi-Z state so that another processor can take control, for example an external ICE, while the main MCU remains in the board. This sounds very similar to a DMA.

Alfredo del Rio.