Write protecting is safe against viruses!
As far as I know, nobody has ever managed to write to a write protected disk unless the little pin in the disk drive which tests for write protection is broken. However that normally causes things to work the other way reporting that the disk is write protected when it isn't!
There is certainly no CIA bits you can set to tell the drive to ignore the write protection tab and write anyway!
You would think if someone had done it they would have bragged about how they did it, so I am 99.999% certain it's impossible.