The interesting, hidden, 'vendor specific' commands are not easy to get to. The people that know them on this forum won't tell you, but luckily, if you work hard enough, you can find them out yourself
These 'secret' commands that nobody will explain to you, vary from model to model.
Sometimes dedicated 'new' commands are used, like the 0xFE for DJSA that you described. Alternatively, sometimes these Vendor commands are extensions to 'normal' (not vendor specific) commands. For example, on most modern Western Digital HDD's, the interesting 'secret' commands are extensions to the 'SMART' 0xD0h ATA cmd. They can be accessed by setting special values in the Feature register en sector/cylinder selectors. But not only the features and the sector/cylinders regs are being used to 'convert' normal ATA cmds to vendor specific commands, sometimes even the drivehead register has to be set differently, to get to the 'secret' code => bit 7, 6 and 5 should always be 1,0,1 .... according to the standard ATA specifications
So you have to think 'out of the box' ...
And sometimes standard ATA's are used, but you have to issue another 'unlock' ATA first to modify behaviour (the 'unlock' command (i believe somebody called it 'SuperOn', on this forum) switches the drive to a different mode). For example, on some HDD's you can read the SA with normal 0x20/0x21 ATA commands, BUT you have to issue the 'unlock' ATA before, otherwise the drive will reject reading the SA. And sometimes a combination is used (for example, the extensions to 0xD0 on WD, won't work, if you haven't sent the WD 'superon' command first)
Anyway, can't help you with the specific DJSA drive, haven't looked into that one, but hope this info is useful to you.