That is correct, SPI Interface not JTAG. But I believe Peter Dannegger had an excellent solution for when the code prevents ISP - force EA to for external execution, bypassing the ISP preventing code.
I have also found the Altera byteblaster can be used to program the atmel 8252 and 53, if wired properly and the software is configurable as to pins.

But, for all this trouble, you still only get 12K of code running on a slow processor.