I posted a similar cry for help at the Neo Geo forums: http://www.neo-geo.com/forums/showthrea ... elp-Needed
The consensus there seems to be that even though I'm getting correct voltage, I still may be without ample Wattage. Does that sound about right?
I even tried a spare PC power supply and a second supergun that I read as outputting 12v and 5v (with 3.3 being present coming out of the I/O), and got the same results. On the tv I see a quick white flash when powered on, then just a black screen. I get this regardless of which sync mode is set.
Granted, with this setup there is still the variable of running the mobo with power routed through the I/O-- honestly I don't know how this is a factor, but again some folks over at the Neo Geo forum have suggested that the power supply in my Sigma simply doesn't have enough 'juice' to power both the I/O and the Naomi.
FWIW the specs on the power supply:

Still wondering whether a Naomi PSU is a good investment to try and arrive at a solution, but at this point I'm willing to shell out another 40 bucks.