Great install! Very impressive.
Since (from what I can discern) the only way to "properly" shut down the mini is via the screen menu selections. So if you don't want to enter sleep when you turn off the ingition you are left with the following options:
1) Go thru the screen menu commands each time you want to shut down the system. You'll then need a "power button disconnect switch" to isolate the un-wanted power button command from the P1900. This would simply be a single pole switch in series with the power button cable/command from the P1900. It is similar to the "valet switch' but on the output side of the P1900 instead of the input side. I'll post a diagram shortly.
2) Write a script that you can execute from a "shutdown" icon on the screen when you want to shut down prior to removing power. This, in conjuction with a "power button disconnect switch" will "properly" shut down the Mac.
3) Leave your P1900 jumper in the "hibernate" postion. This will initially put the Mac in sleep, then 60 seconds later abruptly remove power. Obviously (from the "official" recommendations) not desirable. However, from what some people are saying, they have done this for an extended period of time with no apparent adverse effects.
Anyway, great work. Keep it up!
Edit: modified diagram with "power button disconnect switch" posted HERE.