When I must work on a ZF's innards, I jam a cloth between the ring gear and it's pinion with the transmission in some gear. This locks the gearbox without any tooth damage or complicated mechanisms needed. Note the ZF Shop Manual is only an overview and not very explicit in many areas. Thee are some official ZF factory tools and jigs shown (pg 8-10) to do certain things to the 'box , and all we have is work-arounds discovered the 'hard way'.
Some owners have done modifications inside after finding the price of certain parts too high. I'd be wary of using such a transmission. One owner used liquid Lock-Tite plus peening on a setscrew that retains one of the shift forks. The ZF was setting on end, and he used too much Lock-tite. It ran down the shift-shaft and got into one synchronizer, locking it solid so the box could not be shifted at all. Removing the ZF from his Pantera and tearing it down a second time, then chiseling things loose took all weekend. Be cautious!