I've only ever looked at them from a distance and avoided touching for fear that it might turn me...

Obviously the demo user is volatile, sort of a cross between a guest and a kiosk type set-up.

Can you run BOINC concurrently as a separate user hidden from the "demo" or perhaps as a daemon and avoid the "Demo" account entirely?

You could always boot them from a USB stick or CD loaded with Linux ;-)

I remember being rather impressed with the Mac Pro performance, although I was looking at it from the perspective of my AMD 4200+ X2 at the time.

I don't know about the performance of Mac OS, but note there are some of the most productive SETI@Home machine running it .
It is hard to tell how stripped down the OS might be though if all it has to do is crunch SETI.