Assuming emphatic is indeed in /drive0/var, is executable, and is starting up without any errors, that exact syntax (with one space in between ;@DC and ;@EXEC_ONCE) should work just fine. If it's not, then please attach a boot log (from the serial console) showing what's going on when you boot your empeg in DC mode. Force it into DC mode using Hijack's "force AC/DC" menu option if you can only get a boot log on AC power.