Quick update,

a bit of google-fu took me to the following page:

http://forum.allaboutcircuits.com/threads/auto-off-switch.27626/

I think I have found a neat solution to my power on/off switch:

My version will have the line that goes to VDD connected to the 3.3v converter ENABLE , that will in turn power the Trinket Pro, which will then pull high the BS108G gate to keep everything powered or pull it down for auto shut-off, while monitoring the line currently connected to pin 4 of the picaxe in the schematic to monitor for a turn-off signal.

SWEET!

Cheers,

Mick

EDIT: Both transistors are reaching End-of-life and are not available anymore for purchase... let's look for alternative transistors.