I don't know too much about the red seal system, but it doesn't strike me as something that would be very difficult to forge. I model Japanese workers as being pretty meticulous, but it strains credulity to believe they are carefully authenticating such stamps. (How would you even?) I suspect that system is vulnerable to a similar attack to this one, it's just that the risks outweigh the benefits.

Which also goes for the situation we're discussing. I've never heard of an attack like this and I doubt it is frequently attempted or succeeds.