Thanks again to everyone who suggested potential solutions to my 12 volt trigger question. I ended up with a Niles AC-3 with my surround amp and subwoofer plugged into it. The Niles Trigger In is connected to the "AUX" 12 volt trigger on the Bryston SP2 , and then the center channel Bryston 7B trigger In is daisy chained from Niles Trigger Out.
So operationally, since my L/R front 7B SST triggers are connected to the SP2's On/Off trigger, they sequence on when I first switch on the SP2. Then, having selected "On" for the Aux trigger on "DVD" and "Off" for everything else, when I select "DVD" as the source, the center and surround amps and the subwoofer are switched on. Selecting any other source leaves the L/R amps on, but the components controlled by the "AUX" trigger shut off.
Tech note: the SP2 12 volt triggers have a rated output of 80 mA total, and the Niles AC-3 manual specifies a 100 mA minimum input. Despite the relatively weak output of the SP2's trigger (vis a vis the AC-3's stated input requirement), the configuration described above works fine. However, the AC-3 didn't work at all when chained after the 7B instead of first in line from the SP2 (the 7B came on but not the AC-3 switched outlets). Although the 7B reportedly only saps 2 mA from the circuit, that seems to be just enough to break the chain.
Also, daisy-chaining two AC-3's and then the 7B didn't quite work as desired. Everything would come on when DVD was selected, but then after about a minute everything controlled via the Aux trigger would shut down. Fortunately, further experimentation showed that powering up the sub and the surround amp at once does not trip the breaker, so the second AC-3 isn't really needed. Thus, the configuration described above using only one AC-3.
Burke