I had the same issue. While it can be done (I did it for my SNES case), I had to solder a couple of pins to the powerblock (see this post: http://blog.petrockblock.com/forums/topic/snes-gpio-config-with-power-block/)
Using a different power controller would probably be your best best if you don’t want to solder or do anything like that (and if you live in the US will be cheaper and arrive faster than from overseas).
That said, I have a Mausberry power controller in my N64 conversion console. While it works, it seems a bit more no-frills vs the powerblock. It works, but it’s basic. no LED indication of what’s going on (power up or down), have to solder a pin in for the LED, not a service that runs, just an endless loop script that polls for the switch to send the shutdown command. I like the powerblock better (and it supports the project). The ATXRaspi is another power controller I have in my C64 conversion (yeah, I know) and I like it, but it only works with momentary switches so not a good fit for the SNES/N64 case.