jtag via usb

If you are trying to get easy field upgradeability, you can write to the config. flash device through the Nios processor itself I believe - so that is the ultimate solution since it's soft; you can update through ethernet, serial etc. etc... you just need code to do some simple routines with the config flash.

And I agree with others - I can't see altera giving away that sort of IP since it would undercut their USB Blaster sales (which must be quite good considering the relative simplicity of the USB blaster).