Based upon various sources, the artillery fire can be anywhere from 130 to 140 dB spl at 3m, which calculates to anywhere from 100 dB to 110 dB at the stage and audience areas in your scenario. I've never had artillery fire drowned out by the PA, but then again I mix outdoor orchestra work around 95 to 98 dB measured at FOH.
If Bennett's idea helps you keep peace with the various committees, then by all means do it and bill it. I recommend a Senn 421 for this, as they've been proven to handle it cleanly. I don't anticipate your needing to worry about line losses in the mic cable from there to the mixer.
Edit: Forgot to mention that you'll pick up quite a bit of it in the orchestra mics. You should be prepared to delay the artillery mic if you do actually have to add it to the mix. Thanks to Tim for raising that issue.