I've found a copy and updated the OP with the info. I've using on my Go, and so far so good! You have to enabler "Hacker Mode" in order to be able to write to ef0: -- otherwise ef0: is read only.
The standard version of PSP Filer can't access ef0:/ on the PSP Go, but instead only accesses the memory card. Back in 2012, though, The Zett (from Wololo), posted a video on YouTube showing off a modified version of PSP Filer 6.6 that can access ef0:/. However, all the links of it I've found...