According to the Philips datasheet the value of /EA is latched when RST is released and further changes have no effect , which is clearly something different than "sampled at powerup". This is actually the same behaviour as the "standard" 8052's.

I doubt however that the suggested reset scheme still would be usefull. Although it would be possible to differentiate a power-up reset from a hardware reset, the uC's registers will be reset to default in both situations. This would mean that all your precious hardware presets will be discarded and reset defaults are reloaded once the user program kicks off.

If software protection is what you are looking for, you obviously need to go for a different solution.