I'm not arguing about it working (I strongly believe it will work just
dandy), but I would like to know about failure modes.

Do you get a redundant alternator? or a redundant water pump?

The most likely failure mode that I can imagine is a broken belt. With a mechanical pump, a broken pump belt means an instant loss of cooling. A broken alternator belt could also take out the pump belt, with the same results.

        Last nights tests showed that their is a significant reserve time without an alternator. And that was with the pump running at full power. At cruise speeds I would expect less current draw due to the controller slowing the pump.

        One observation I did make when I shut down the pump last night, was that the motor was quite warm. Not alarmingly hot, but enough that it was uncomfortable to hold your hand on it.

        All things mechanical can be expected to have a limited service life, often with degraded service preceding total failure. Electrical devices in theory should last indefinitely (excluding bearings), but in reality do not and usually give no warning of their failure.

        In the case of a broken belt, I feel this is a safer system. A spare pump could be used and still be lighter than the mechanical pump, however this may be a little excess.

S. Todd Bartrim
Turbo 13B rotary powered
RX-9endurance (FWF)
C-FSTB (reserved)
http://www3.telus.net/haywire/RV-9/C-FSTB.htm