While at B-Rev's in Philly Fuzzy showed me these cool thread on protectors for the non drive side of your flip flop track hub which will protect the threads from abuse during grinds
It's a good idea and a nice product.
There is also a free recycled piece that you can use for the same job
Just go down to your local shop and ask if they have any Shimano Centerlock covers.
These come on Shimano Centerlock disc hubs and will fit right onto the freewheel or cog threads on your hub. While it's true that it likely won't do as good of a job at resisting impacts as the metal piece, it's free. So there you go. Better than nothing.
They also do a great job of just keeping dirt and grit out of your threads if you've got nothing threaded onto the opposite side of your hub.