UPDATE!! 
OK... I guess Alzheimer's disease is starting early, because it turns out I DID throw a "splash" of Prestone into the cooling system after my last outing. 
(I honestly couldn't remember doing this.... Must have done it in my sleep.... 
) Not enuf of the "green stuff" to protect it down to -40F, but enuf "green" was present to protect it to approx 10F (No, I didn't eyeball it... I used a tester).
Just to be safe, I drained the system and refilled it w fresh Prestone 50/50. 
Now "Mother Nature" can't ruin next year's racing season....
(Of course, I'll have to drain out the "green stuff" before the next HSR or SVRA event, as "water only is the rule on the track...)
STILL CURIOUS to hear if anyone has experienced freeze damage to their Spridget. IF SO, I'd love to know if the "freeze plugs" (Ok, I know that they re REALLY just "core plugs", designed to fill the casting core access holes...) popped out, or what broke if they didn't. Your costly mistake might be a motivator to others to protect their engines.
ALSO CURIOUS to hear if anyone has experienced freeze damage to their car battery. (A car battery loses it's ability to resist freezing as the charge diminishes...)
"Cheers!" 
-Bear-