Hmmm...interesting this one.

Just looked at the datasheets on my PC for 16F84, 16F628 and 16F628A

The 84 doesnt show diodes but mentions them and specifically says that RA4 is open drain and only has a diode to VSS. It also says that the maximum voltage on any pin is VDD +0.6v except MCLR wich is 14V

The datasheets I have for 628 and 628A both SHOW a diode on RA4 to VDD but also show it as an open drain pin. The spec says max voltage on any other pin is VDD + 0.3V EXCEPT MCLR AND RA4 which are 14V

Obviously something is not right in the datasheet and I would suspect that the diod ISNT present. My understanding of having an open drain pin is specifically to allow it to sink a load driven from a higher voltage hence the electrical spec stating 14V maximum. If the diode IS present then you wont be able to do that.