All commercial two-way radio power leads have fuses in the Pos and Neg leads.
This is to protect against reversal of power leads when the aerial is
connected - there is then a path from the Pos Battery, through the negative
lead, the radio chassis, antenna coax shield to vehicle earth !!!

Adding a Zener diode provides no protection here ! The zener is useful for 
overvoltage - but is not needed in the FT817 for reverse protection. There are 
internal series diodes in the power (D1085)and charging (D1083) circuits to 
isolate any external reverse voltage. 

The battery pack is also connected via a series diode (D1084). On current 
production there is now a PC-board mounted 3A fuse, as well as a plastic shield 
attached to the battery case to prevent batteries shorting to the case while 
being inserted.

Mike VK2BMM