Can a secondary or additional core be loaded and made active (powered on) without via user application or a boot loader
Assuming the boot code has at least the same privileges as the kernel it is booting (Most likely the case) then there is no reason why not. 

Userland does not after all have direct control over such things, there is always a call into the core 0 operating system to set such things up, so there is no reason the core 0 kernel cannot load a program into one of the other cores on its own behalf. 

It is likely (but by no means certain) that the core 0 kernel will explicitly put the other cores into power down on startup if they are not being used, so while a bootloader or a shim probably could load and execute code on the secondary cores, it will likely be killed once the core 0 kernel starts up.
It is interesting to speculate on having a shim load code into core 1 or 2 that then tries to patch the core 0 kernel image before it is executed to avoid being killed by the core 0 os.... 

Regards, Dan.