Flying through cloud without an IR (IMC in UK) is never legal.
Come to think of it, surely a forced landing is breaking the law as you don't have the land owners permission. In a Mayday (EF)situation you do whatever it takes to save life and limb. The donkey stops and you've no choice but to go through IMC - who cares about the paperwork at that stage? I don't think it's a reason not to go VFR on top though. All PPL's do some IMC traning for 'Emergency Use' only.
Thus in a single engine aircraft, and JAA PPL, flying above 8/8 Overcast cloud cover would only be legal up to the point where the engine failed and flight in IMC became imminent.
Apart from the fact he has to stay 'in sight of the surface' so he'd breaking the conditions of his license long before then.
BTW, my 4/8 came from the ICAO definition of a 'cloud base'. Not saying that is or isn't classed as the 'on top' but I guess the 'on top' may be 'on top of the clouds' as in 'above the cloud base'.
Dunno really...