This morning I received an answer from Tim regarding yesterday's inquiry into wiring the LM-3 to a keep-alive or a battery, and my question about when he thought Ring's keep-alive would be available.
On running on battery:
"To connect a battery to a LM-3 it would be best to connect the negative 
battery wire to the 'negative terminal' on the 6 pin connector and the 
battery positive to the red wire on the 9 pin connector."
For me at least, since I have a steamer that I plan to run fully on battery power, and given Tim's answer, I think I'll stick with connecting the battery leads to the rail leads (red and black on the 9-pin) and not mess with the new negative lead.  That way all I am doing is replacing the DC supply that normally comes through the rails with an onboard DC supply.  I get that connecting it according to his suggestion probably eliminates one forward voltage drop (approx 0.65V) across the internal rectifier, but I wonder what else it might bypass (see next quote).
On using an existing DCC keep-alive:
"We do not recommend connecting other manufactures products to ours.   
You could damage either our product or theirs and the worst case would be 
that you cause a fire.  However, at your own risk, you could connect a 
DCC keep alive to a LM-3s since it has a negative terminal.  If the DCC 
keep alive can withstand track voltage you could connect the most 
negative wire to the 'negative' terminal on the LM-3 and the positive 
wire to the blue wire on the 9 pin connector.  Connecting to the blue 
wire will bypass some of the LM-3 internal circuit protection and can 
cause more damage to the LM-3 if there is an internal short (not likely 
- but not impossible either)."
This leads me to the conclusion that I will wait for Ring's keep-alive product.  Using theirs, unforeseen problems will be covered by Ring.  I can't afford to replace LM-3S modules because I ignored the risks.
And on the subject of Ring's keep-alive:
"We do plan to make a RailPro keep alive but it is not expected to be 
available real soon. "
I am bummed that the keep-alive still doesn't have a target release date, and the release of said product seems to be getting pushed off further and further.  Any hope I had of it coming out soon is pretty well dashed.  But, they say patience is a virtue.  Sometimes, I'm not terribly virtuous.   
