So I wanted to put an AT motherboard into this nice ATX case I picked up. To convert the momentary front power button to a continuous on/off signal for the power supply I wrote a little script to run on an attiny85. In case its useful to others it's here I put it up here:
https://bitbucket.org/LauraJones/at-atx-power/src/master/

I built it on some protoyping board so didnt bother designing a PCB. I added a PS2 adapter for the header on the motherboard and put it on a blank ATX cover.